This one is going to be out of the price range you specified, even more so if you want to get it fitted out a bit more for diving:http://www.trademe.co.nz/motors/boats-m ... 649861.htm
The Auckland Uni Club has a model a couple years newer and we absolutely love her. Great little boat for diving, we can get 6 divers on there plus skipper and first mate for a two dive day quite comfortably. Shane and Julia have bigger models up at Northland Dive as well.
I have done pretty much all my small boat diving off of RHIBs, and very much prefer them for diving. The only things I miss is the room you can have on some of the really big boats for your gear bag so you feel like less of a nob when you forget something in it.