Get a proper spearfishing suit. I have lots of customers who initially think they can get away with a surf suit, they all come back for a real suit in the end!
5mm, unless you are planning on long dives in the depths of winter, in which case a 7mm jacket is nice as an extra. I find high waist more comfortable than long john. Made to measure is well worth the money
The only company still making any suits in the UK is Snugg in Newquay. They only do (very nice) surfing and triathlon suits, starting at about £200
For a first suit get nylon outer/ cell inner. I like camo, but I dont think it actually makes much difference most of the time. Dont get too hung up on neoprene types, bear in mind that the manufacturer of the suit knows a lot more about neoprene than you do!
I may be biased, but Elios really do make the best spearfishing suits in the World!
cheers
dave
www.spearo.co.uk