As Graeme says, it's very difficult to value a RIB without seeing it -- however, if it's in decent condition then this doesn't sound like a bad price.
You are right to be concerned about the engine though, and unless you know your way round outboards I would suggest that you get an engineer to check it over for you.
The condition of an eight year old engine can vary wildly. If it has been looked after it should have plenty of life left in it. If it has been abused or neglected (possibly the more likely option!) then it could give you no end of problems.
If you can find an engineer to check it over for you, then it would be a good investment of a few pounds. You shouldn't really expect any comeback if you do
get problems, but it should help you avoid buying a complete lemon!
Fitting extra seats is not cheap unless you are able to make them yourselves. A single jockey seat will set you back about £400, with double bench seats (not really recommended on a small RIB anyway) from about £500.
Outhill manufacture seats and consoles used by various manufacturers and sell direct too. Details at http://www.outhill.com/general.htm