.you're going to get a very nice example of most decent R.I.B makes around the 5.0 mtr mark with say something like a 70hp and above on the tail. Anything less (IMHO) just ain't worth having once you get into serious ribbing.
If you want to go longer, say 5.5 / 6.0 mtrs and above then unless you're willing to go for something old, tatty and of course dated in technology I'd either reconsider your budget or, if it were me review many of the threads on here which advise that sub 5.0 mtr RIBS are more than capable.
I'd agree with the above comment that the longer the better / smoother / more comfortable, however having owned both 4.2 mtr (Zodiac Yachtline) and 4.8 mtr (Zodiac Medline) and now 6.0 mtr (Medline II), I can advise, with experience that having cruised with bigger 7 mtr Prosports ans smaller 4 mtr Seariders, we have all left and later arrived at the same point enduring the same conditions (force 7/8) safely and at the same time. Yes some wetter than others but nether the less safely and in reasonable comfort.
I'd advise you to seek out a cruise where you'll no doubt have opportunity to jump aboard a number of different ribs.
Take a look at www.boatsandoutboards.com
which is possibly the very best U.K. site for secondhand RIBS. Further, this forum contains a great many very experienced RIBsters who are more than willing to assist. You're always welcome in the North West.