Ok lets start with one item at a time as there is much to cover and once we have been through each item and you want to ask any questions then please feel free to do so.
We will start with the Speargun this is the one with the most variables.
There is the option of a closed muzzle or open muzzle Speargun if you click on this site
https://www.makospearguns.com/
And then click on the HOW TO INFO then look to the far left it will say How to rig a closed muzzle speargun and how to rig a open muzzled speargun and will give you diagrams which will be much better than me explaining it.
There are non railed and railed spearguns (a railed speargun means the spear sits in a rail so the spear fires out with less wobble so the spears trajectory is more accurate,just think of a crossbow it has a rail to it to so the bolt shoots straighter ,glue on rails can be purchased for a non railed speargun if you wish to do so (Marine glue would be the best option for this)
Aluminium spearguns,wooden spearguns,carbon spearguns and it all depends on how much money you want to spend,what type of spearfishing you want to do and the water conditions(if the sea is always murky where you dive then there is no point in a 120cm gun as you will never be able to use to its full potential so are better off getting a 75cm or 90cm speargun.
If you dive in a area with heavy Kelp a shorter speargun is a better option as you would not be able to move a longer speargun around so easily so miss prey.
If the sea is clear where you dive then you can look at a longer speargun which will shoot a further distance and or has more power for taking down bigger fish. The Carbon speargun being the lightest so there will be not so much hand and arm fatigue with holding a Carbon speargun for long lengths of time but they are more expensive.
Pneumatic spearguns and roller spearguns are both guns that for there size will have more fire power,I personally have never shot with either of these although the opinion of many on the Pneumatic gun is that if you are using it when diving in a sandy area the sand gets into the working parts and then the gun packs up. Basically with a standard speargun there is less to go wrong and parts if required are easy and cheap to replace.
There are different diameters of spear 6mm,6.5 and 7mm etc and usually a speargun will come with a certain diameter of spear which works best with that particular speargun and again it depends what type of spearfishing you want to do if you are going to spearfish smaller fish a 6mm spear will do if you want to spear bigger fish then a 7mm.
Spears also come with either notches or sharkfins (I have never used the Shark fin spear as i think that the notched spear is more streamline).
Spear shafts with breakaway or Slip tip heads so there is less chance of damage to the spear shaft.
Different types of types of metal the spear is made of, Stainless steel,Tahitian and Spring steel the spring steel being the hardest material so if your spear hits a rock it is less likely to bend and also keeps a sharper point as it is a harder metal than Stainless steel. A spear made of 17-4 stainless steel is also a good option for quality and rust proofness.
Always take an extra spear or two as if you loose one then your days spearfishing is over it would be like taking one hook if you were to go fishing with a rod.
There are different diameters of speargun rubbers the thicker the diameter of rubber the more power it will deliver but requires more strength to pull it (some spearos prefer a smaller diameter rubber as they can load the speargun quicker so not miss the opportunity of fish coming into there firing range (I have found that the Cressi rubbers are easier to pull than the Beuchat rubbers so i can cock the speargun quicker depending on where i am shooting and what fish i am shooting) You can make your own speargun rubbers from bulk rubber and can find videoes on the internet on how to do so.
There are different shapes of rubber one type is a U shaped rubber which delivers more power than two screw in rubbers as the U shaped rubber carries the spear further up the barrel of the speargun also you can use two rubbers on some spearguns but the speargun needs to be made solid enough to take the strain of those two rubbers.
You can use either one or two rubbers two rubbers giving you more power the first rubber being the shorter of the two is pulled to the first notch the longer to the second notch or you can just use one longer rubber to the second notch which will obviously give you more power than one shorter rubber.
Different wishbones (the wisbone sits in the notch of the spear once the rubber is cocked) there is the Articulted and Dyneema wishbone.
The Articulated wishbone is an all metal construction wishbone some are just a bit of wire which can fracture easily and do damage to your hands other are more solid in there construction so safer. The Articulated wishbone is also a noisier as the metal wishbone as it clinks against the metal spear.
The Dyneema wish bone is made of what it says it is Dyneema and is a quieter wishbone when cocking the speargun as there is not metal clinking against metal like there is with the Articulated wishbone so the Dyneema wishbone does not scare the fish you are looking to spear (Spearfishing is all about stealth so the quiter you are you are more likely to spear a fish) The spear notches need filling down with a needle file when using Dyneema otherwise the sharp edges will cut the Dyneema. (some spearos prefer the Articulated wishbone as it is more reliable than dyneema everyone to there own as everyone is different)
Thats enough break time lol.