Here is my 3cents on the original Q's ...
What should one look for in a kayak?
Tom was right about stability, remember you need to get on and off the ski easily. And on occasions you will find your self doing some yoga to get your suit top on or off.
This said speed is also a big thing for me, a slow painful to paddle ski is just not worth the effort. No point in getting to your location half dead. So it needs to be quick.
Storage is also very important. You need compartments for gear. weight belts and masks dont mix. and spearfishing you have fins ect to carry so the hatch needs to be big. A huge bonus is if the speargun can go in the hatch.
Plastic or fibre?
I owned a plastic C-kayak for many years and despite its impractical hatches I worked around them. In its day it was faster than any fishing ski ... they were so far behind! That's why I got it. Now there are many glass skis much faster and paddle better. Here is the thing a plastic ski is virtually indestructible. I even paddled mine down a white water course and down 12ft waterfalls!!
Glass skis have a life span and some are notorious at being weak. But that is the only good quality of a plastic ... it will out last you
Considerations when using this type of equipment?
Tom laid out some important point on using a ski in his previous post.
Only thing I will add is that don't be delusional and think that you will be off to Mozambique and go and clap the fish on a ski.
A ski is a fair weather vehicle and in Moz you have strong currents to deal with as well. This means long fast drifts , with long slow paddles back up to the starting point. And if the wind is blowing as well ... then its even more difficult.
This said when the conditions are right I enjoy diving off a ski, its quiet, mobile (like Tom said) and I enjoy the solitude while I quietly pick apart my favorite spots with competition from others. (Yes I know not the safest practice..)
Legalities surrounding the employ of kayaks for spearing and the like?
Its just no bugging and you also need a fishing permit to 'fish' off a vessel. Available from post office with other licenses.
Are there any reviews on kayaks available? (If not,will there be in future editions of Ultimate Spearfishing?)
Hope to do some reviews in the future ..
Any other suggestions or points I have not yet considered.
Heed Tom's sentiments (post before this one)... they may seem harsh, and maybe a little over reactive, but there is truth to all of what he has said.
If you don't fit the experienced profile it does not mean that you should not get a ski. Just that you need to be seriously realistic. Maybe take the ski on a couple light fishing trips when the conditions are good and get to know your ski and how to handle it. You are going to get drilled in the surf and .... you will learn some lessons the only way you can learn them and that's the hard way.
If you are already comfortable on a ski and have that down, then start you ski diving at a place you know well and feel comfortable, and again in good conditions.