Here are some suggestions.
1. Read the features section on Ribnet - there are some articles on what to look for when buying a rib.
2. Read the articles on RIB International's reading room, here: http://www.ribmagazine.com/index.htm
3. Read some boat tests - RIB international have a lot online
4. Do a RYA L2 Powerboat coarse
5. Browse the Ribnet forums - the search facility is very useful.
6. Ask some more directed questions back here
7. Go to a boat show
I recommend you do all of these before buying a boat, since you'll be changing your mind about what you actually want as you go. The most important question is, what do you want to use the boat for? (With whom? In what conditions? etc etc etc) Take your time - in my experience it takes a while from a standing start to work out what would be suitable (but it's fun doing so).