There are probably more theories and ideas about speargun design than any other piece of kit ranging from the brilliant to the absolutely ridiculous.  Here we will try to cut through some of the bull and get to the main points.

Here are a couple of things to remember:

1.  Guns always shoot best rigged up as they come off the shelf.  Good manufacturers spend a lot of time and effort testing their products so it is logical to assume that they have rigged their guns with the right length rubbers and shafts. 

2.  If you want more power or distance you need more length.  The most important thing for the range and power of your gun is the distance between a slack rubber and a tight one ie the barrel length.  Simply adding more rubbers is not going to help.

3.  If you miss a lot of shots or are thinking you need a double wrap you are probably shooting from to far away.

4.  You cannot save money by buying parts and making your own gun.

The speargun can be broken down into a few main parts, which we will talk about individually, that all need to be perfectly balanced for the gun to shoot straight.  When looking at a gun ensure that all the bits seem to fit properly and that the shaft is sitting perfectly straight.  It is very common for lesser brands to simply buy discount, mismatched components from other manufacturers and assemble them under a different name.

Think about what your main targets will be and buy a gun accordingly.  In NZ the best lengths are from 110-130.  If you don't usually target kingfish or larger pelagics it is probably pointless buying anything longer than 110 - If you're only targetting kingies buy a 120-130.  Remember that you have to be able to see the end of the gun so if you're usually diving in limited vis don't buy a huge one.

MECHANISM AND HANDLE:  The mechanism is the most complex part of the gun.  It contains nearly all the moving parts and is usually the first part of the gun to fail.  Look for simplicity and big, chunky parts.  Will the mech accept a wide variety of shafts or only a certain kind?  When you load it does it make a satisfying clunk or is it really tight?  Does the shaft fall out when you pull the trigger or do you have to pull or wiggle it?  What kind of springs does it have, big solid, stainless ones or silly little steel coil things?  How does the line-release work?  Don't worry too much about the safety as you'll probably end up ripping it out anyway.
The handle of the gun must fit your hand nicely and feel comfortable.  Also look at the material used.  Nylon, or even better glass-reinforced nylon, is much stronger than plastic.

BARREL:  After the gun length, probably the most important thing is the rigidity of the barrel.  A weak barrel leads to flexing and a crooked shot.  Strength is determined by the quality of the material used and its wall thickness.  Guns should have between 1.8-2mm wall thickness. 
The other major thing that increases rigidity is an extruded rail like on Rob Allen guns.  This rail stiffens the barrel considerably and is the main reason for having one.  There are many imitation "railguns" out there that are just a normal, thin, aluminium tubes with a plastic sleeve over it with a rail.  Obviously this adds no stiffness to the barrel, but it actually also reduces the life of the gun as water gets caught between the sleeve and barrel and eats away at the aluminium.  The rail does guide the shaft to a very limited extent and makes the length of the spear overhang less important.
It is very important that the barrel is water tight.  The best way to do this is to plug each end with a rubber bung.  O-rings around the muzzle or handle, or foam is far less effective.
There are three main materials used for barrels, aluminium, carbon and wood:
Aluminium is by far the most common.  This is because it is light, very strong and cheap.
Carbon guns are becoming increasingly popular as they offer the same strength with less weight.  The ends of carbon tubes however are susceptible to cracking and you should never buy a carbon gun with a male-fitting muzzle or handle.  Beware of buying carbon tubes not specifically designed for spearguns as they will often soak up water.  Carbon is considerably more expensive than aluminium.
Wooden guns are generally twice the price of modern aluminium ones for no benefit performance wise.  They do however look very pretty.

MUZZLE: There are a few main muzzle designs but with all of them the most important thing is the material.  Nylon is best as it is strongest (stronger even than aluminium ones which seem to explode after a couple of years).
One of the main differences is whether it takes screw-in or loop rubbers.  Screw-ins are possibly more convenient to change but have more failure points whereas loops are simpler and generally cheaper.
The other major difference is between open and closed muzzlezs: 
Open muzzles rely on the shooting line to hold the shaft in place.  They are a cleaner design and have a very clear line of sight when aiming.  They also have less shaft slop making them quieter.  The rubbers do not however pull in straight line so you should only have an open-muzzle on a railgun.  They are also more complicated to reload.

Closed-muzzles hold the shaft in place.  They make re-loading very simple and allow in-line rubber pull making them suitable for tube guns.  They are available for loop or screw-in rubbers.  They do obscure the end of your spear when aiming.
At the end of the day no muzzle is better than the other - except in terms of construction - and it's up to you which you prefer.

SHAFT:  The business end of the gun.  There are two main materials used, spring steel and stainless.  Spring is much stronger and the shafts last longer despite the little rust that the do acculmulate.  Stainless is lighter and faster.  Generally, in NZ ,spring is far more suitable as we shoot bigger fish than the European ones stainless shafts are designed for.  The flopper should be at least 80mm and flop the first 45 degrees easily then jam open.  Some people prefer it on the bottom of the shaft so that gravity helps it open and others prefer it on the top so that it lies flat while travelling through the water and is less visible to fish.
Shafts come in various diameters but the most common are 6.5,7 & 7.5mm.  As a general rule guns shorter than 120 should have 6.5s, longer one's should have 7s and double rubber guns should have 7.5s.
It is important to think of the balance of a speargun when selecting shaft and rubber thickness. There is a thought out there that shafts and rubber is like beer - 6 was good so a dozen must be better. This is not so. I am forever talking to people who have put 2 x 20mm rubbers on their 1.1 meter gun with a 6mm spear and want to know why they cant hit anything.   Or worse tell everyone who will listen that there gun is a piece of crap because they cant hit a fish.
There has been a move in the big gun world towards ever thicker spears. Rob Allen do a 7.5mm but many manufacturers are now doing 8mm or bigger.
You often hear things like " everyone knows a thicker spear has moreimpact " Well does it ?
Here are some numbers that represent the relative mass ( weight ) of different diameter spears. Everyone knows that a thicker spear is heavier , hands up who has ever worked out how much heavier

                               Shaft Thickness                                   Relative Mass               Shaft volume  =  radius squared x 3.141 x length
6.0 mm                                                   28.27
6.5 mm                                                   33.17                                   x specific gravity. steel = mass
7.0 mm                                                   38.5
7.5 mm                                                   44.17
8.0 mm                                                   50.25
9.0 mm                                                   63.6

See how the weight of steel in a shaft increases very quickly when diameter is increased a small amount. An 8.0mm spear has 30 % more metal in it than a 7 mm spear of equal length. All the better to hit the fish hard with I hear you say.
How hard you hit a fish depends on the amount of K ( kinetic energy ) you can deliver into it skull. And K is worked out like this

 K = m .velocity squared                  that is weight multiplied by ( speed x speed )

So if you are a maths genius you will see that the speed is far more important than the weight in working out K. And how do we get speed ?
With acceleration of couse , and acceleration is worked out thus
A   =  force / mass
So our 8 mm shaft with 30 % more mass will not accelerate as fast so will be slower thus will achieve much less K. This is why Porches go like hell and Mack trucks dont - because they are dragging  20 tonnes around and weight has a drastic affect on acceleration.
So a bigger shaft needs more rubber to throw it. But then you run the risk of barrel deflection and the resulting inanacuracy
The lesson is becareful when changing bits  around

Guns should have about 40cm of overhang.
If in doubt just put whatever it came with on.
You should have as few notches in your shaft as possible as this is where, on very, very rare occasions, they snap.  Fin tabs are even better.
Slip-tips are fantasic for very large fish as they make a much larger flopper for the fish to rip off and prevent the fish from generating any leverage and bending your spear.  They are however a lot more time-consuming to use and are generally too much of a pain in the backside for everyday use here.

RUBBERS:  Rubbers come in so many shapes, colours, sizes and flavours it can be hard to know what the right one to use is.  Basically theres two types, natural and synthetic.  It's up to you which is better.  Colour is completely irrelevant.
Then there's screw-ins versus loops, again it's up to you which you prefer although it is often more economical to use loops you can make yourself.
The length and diameter is more important. 
Generally you use single,16mm rubbers on tube guns and guns shorter than 110 and 20mm rubbers on longer rail or carbon guns (20mm rubbers are usually too strong for long, aluminium, tube guns)
You can use double 16mm rubbers on guns 120 and up.  This will give the gun a little more oomph and make it easier to load.
Generally the rubber length is between 1/4 and a 1/3 the length of the gun barrel.  The shorter your rubbers, the longer they will be guiding the shaft and the straighter your shot will be.  That is why it is not usually a good idea to put fat rubbers on short guns as they have to be too long for you to be able to load them.
It is also worth noting that your spear will reach a certain speed and then no amount of rubbers will increase that so you're just wasting energy and potentially damaging your gun.
If you do power up your gun you will need to put a heavier shaft on it.
Again what it came with is probably best.
Dyneema is by far the best wishbone material as it is light yet incredibly strong, cheap and finger-friendly.

SHOOTING LINE:  There are really only 3 options for shooting line, or 2 good ones; mono, dyneema or some sort of string.  Mono is strong, can be crimped, doesn't tangle or twist easily, abrasion resistant, nicks are easily visible and it is cheap.  Dyneema is extremely strong, can't be crimped, twists easily, can be abrasion resistant, the core (the important bit) is hidden and it is relatively expensive.  String is crap.

Basically, if you plan to shoot anything substantial (larger than 5 lbs), then you need a band gun.

Polespears or slings lack both the power and reach to shoot anything but the smallest and least afraid (read Triggerfish and Spadefish only).

Pneumatic guns you must load in a single action, so the most power you can get out of one is the amount of strength you have in one push of your arm. I am considering a pnuematic gun for shooting Mangrove free-diving during my surface intervals...

With a band gun, you load 3 consequtive (sp?) bands, using either your hip or belly as a backrest, with both hands. This equates to 4-5 times the power as a similar sized pneumatic, and twice that if you get a longer band gun than the pneumatic.

As to length of gun, you'll never be sorry for having a gun that's too long, assuming you can reach the bands to load it. The longer it is, the more power and reach it has. It SUCKS to shoot a fish near the end of the cable length and have the spear bounce off. I've NEVER had that happen with my Riffe.

Most anyone can load a 54" gun. I would recommend a 60" gun if you're 5'10 or taller. Another benefit of longer guns is cable length. If you use a 48" or smaller gun, you will need 2 wraps of cable to have enough reach. On a 60" gun, 1 wrap is generally enough. Having only 1 wrap of cable simplifies loading without tangling up all the cable.