Personally I don't believe in getting too hung up on NADA, You can spend months shopping in the NADA value range to just find one money pit after another. To me a well cared for, well maintained RV with an asking price far above NADA can easily be worth the money.
As to what fits your needs, you need to look at each unit in person, think how you will use it, is there enough seating, with most Class B's and C's the front 2 seats are useless when parked, on a few they swivel and can be functional to a degree in the living space vs class A's where the swiveling front seats are generally in integral part of the social seating plan with full height ceiling above them.
Personally my cut off points are nothing built before 1996 (the introduction year for the OBD-II diagnostic port, and shortly after the mandating of R134a refrigerant for vehicles), preferably something built after 98-01 depending on Ford vs Chevy chassis (I think 98 was the introduction year for the Ford 6.8L V-10, and 01 was the introduction of the Chevy 8.1L V8).
As to length a standard full size parking space like those found in big box store parking lots are 9x22 ft, anything up to about 24 or 25 feet will fit in one without too much overhang.
