Well I'm sure those pictures, luverly as they are, will be of bugger all use to Firkin Diver in his pursuit of a dive rib!
< Edit -- I moved them to a new thread JK >
In an effort to get back on topic....
Suggest you read the some of the posts on the commercial forum. Sounds like with the fuel tax reclaim petrol outboard is the way to go so you are on the right track!
Ignore all second hand ready fitted boats with lots of nice seats. What you need (of course you know this) is lots of nice deck space, bottle racks and nice comfy tubes for divers to sit on.
7.8m Ribcraft supposed to be good boat. Garygee has one (albeit diesel) so he can comment. Imagine it would be good teamed up with 225 Opti or larger. (Theres the whole 2 vs 4 stroke debate to be had of course.)
Other thoughts might be a big 'umber (8m?), Tornado, mebbe an Osprey or one of those nice Polish BW...sorry Parker 7.5ms!
Or if you are well wadded up and want your customers to experience the best a 7.5-8.5 Scorp!
Probably would go for a major make and avoid some of the ahem, more leisure orientated budget ribs so that it stands up to the commercial use.
EDIT Ooh, just noticed that I now appear to talk sense as well as being a salty old seadog. Thanks JK!