Hi there
Just to say that as a relative newbie, you've made the first step by finding this fab forum, there are a load of ribbing experts that can all help you, whether about the dimensions of a specific boat, the electrics, you name it they really helped us out.
As we are fresh from buying our first rib, I would suggest a number of things:-

work out a budget
remember that once you bought your boat:-

where are you going to store it long-term, for the summer? Do you really want to trailer it to and from the coast every weekend - it gets really boring driving at 60 m.p.h. trust me.

harbour dues

fuel costs
It is amazing how much more you end up spending up-front so factor that in to the equation.

Don't underestimate the cost of repairs to a trailer - everyone talks about the outboard/inboard being the biggest part of the cost, but the trailer can suddenly cost a load of money that you were not expecting (not that we are talking from experience - oh yeah we are!!!

). It is hard to inspect the bearings or brakes.

Having said all that, it is great to have the boat and we love it. We joined a group from here on a recent trip to Priory Bay and the support we received was really lovely.
Remember that if you are talking about all those people coming on board, you need lifejackets, there was a thread on here talking about cheap lifejackets. It is really worth using the search facility on boats, newbie, RIB Virgin (where we received loads of help!), etc.
Good luck with the search.