Hello Rich, you can buy a full conversion kit for the Revell 1/72nd Gato class from http://www.caswellplating.com/models/ , they sell the Subdriver which is a full water tight cylinder complete with motors , ballast control , etc all you have to do to it is add your own receiver as far as I know. Their full package of kit , subdriver , fittings kit ( fore plane mechanism , cast propellers , aft planes and rudder , etc ) and DVD instructions work out at $634+shipping so i’d guess you would be looking at less than £400 with a minimum of a 4 channel 40Mhz TX unit on top to get into the water with a fully working model.
I’ve seen the Merriman "Cabal" reports on the development of this kit into a fully working static diving RC model and provided you follow his instructions this will work and would make a good first sub model for an experienced model builder.
Have a look at the Subpirates website ( http://www.subpirates.com ) or the SubCommittee site ( http://www.subcommittee.com ) , to view all the very detailed reports on converting subs on the subpirates you have to join and subscribe but the advice you will get will be second to non.
Davie