That sounds about right to me -- a RIB that size would certainly be up to the job but I wouldn't go any smaller. I suggest that you fit the largest fuel tank that you can though, as boats this size often don't have enough fuel capacity for serious cruising and refuelling from jerry cans at sea is a real pain!
If you can stretch to it, a RIB a bit larger will be more comfortable and you're less likely to outgrow it in your first season . . .