mate this would deppend on a number of things, where they intend on diving, what they intend on chasing etc..
if they have access to a boat and frequent the the reef i would say between a 1.1-1.3m
if only doing costal stuff in dirty water you would go back to a 0.9-1.0m
Everyone has there own oppinions on brands but if you stick to the well known ones you can't go wrong, rob allen, freediver, aimrite, etc. all these guns are railguns which is pretty important as it makes a big difference in accuracy and loading. If they are serious about it i'd recomend forking out a little extra and get something decent that will last them for a while as with spearguns you definately get what you pay for and you won't have to upgrade in 12months time.
feel free to ask anything that i have missed..
Nidrac