Depends on exactly what you're comparing. A modern class D will be more power efficient and pack more power into a smaller area, be cheaper per watt (if all else were equal which is seldom true in audio land).
Whether you need enough wattage to go with class D in a small size, probably not, depends on how small the size, large the room, efficient the speakers.
The speakers themselves are going to determine the most sound quality difference, though there are certain music genres that lend themselves more to the (artificially) lively harmonics of class D and some that are better suited to the smoothness of class A/B. Some of the class D liveliness will be muted if the tweeters aren't up to it (for better or worse).
The thing with picking up the old designs is you're looking at used gear right? Some may be in bad shape or aged poorly, speakers may need new drivers in them, or for example many with a CD changer will have the changer fail, but on the bright side I picked up a micro system with that fault a few years back (didn't need the CD changer), won it on ebay for $0.01 with local pickup. I felt bad and gave the seller a few bucks for gas money to meet me.
Don't get an old one with a VFD (vacuum fluorescent display), as it will be dim or at least fewer years until then. Then again if it's cheap enough and/or you're only using it as an amp for a bluetooth connected source, an amp doesn't necessarily need a display.