I think you will be surprised by the motor on that bike. The RM 80's have incredible engines. Also, from '96 to '01 the RM 80 is "Bold New Graphics". The acceleration and beefy powerband will simply amaze you after riding that '91 KX, especially if that RM has a fresh top end. The engine was way ahead of its time-case reed, flatside carb, exhaust powervalve. The rear suspension is nice, fully rebound/compression adjustable, but the forks are a little outdated, being unadjustable and the old type. I don't think the RM80 had USD forks until '02. It does handle well, and seems to soak up bumps fine, just be aware you will have to dial it in with oil weight/level or new springs. I recently got my son a '96 and it is identical to a friend of ours' 2000 except for the plastic and graphics.
Eric