The Anglers Afloat Team are due to be doing a SOT safety / training day in Exmouth, have a look on their site. Going along to an event like this, even if not participating, you'll be able to see plenty of different yaks and speak to some of the experienced guys to assess what kit you may need.
It all depends on where you will be fishing , the type of fishing, distances to be travelled (on the water), you're physical size etc. The list of yaks being recommended will nearly always come down to:-
Ocean Kayaks - Scupper Pro TW (faster but less storage space), Prowler 13(good all rounder), Prowler Trident 13 (if you want fishfinder housing , rod pod etc.), Prowler Trident 15 , Prowler Big Game(very stable and buoyant , so suitable for the bigger paddler but also a slow yak)
Wilderness Systems - Tarpon 120 , 140 or 160 . The 140 is probably the best allrounder of the three and is a fast yak, the 160 is fastest but is 16' long at the end of the day. Wilderness Systems Boats tend to be more pricey than like for like from Ocean Kayaks.
Regards a fishfinder , you won't need one to start with , you may decide you don't need one at all , but they can be useful if also a GPS combo as you can store your marks etc., they can also aid in navigation ,but can never be relied on as a safety device , as being electronic they can fail. Your money would be far better spent on a VHF radio and training course to learn the protocol and obtain your license for use.
There is a list of essential safety equipment somewhere on the Anglers afloat site - have a read - it has been developed together with the RNLI and will give you a good idea of what t budget for.
(Plan on around £1k including the yak for starting off and you won't be far out)
Hope this helps
cheers
Wal
