This answer depends on the brand and the level of support. I'd pick up the phone and talk to the company what you are thinking of buying to make sure they will still work on them.
Pass labs, Parasound, McCormack and others I would not hesitate at all to buy an older unit since I know I can get service (caps replaced) if needed.
Mark Levinson, Krell, Classe no way, at least not now. I've been down that path and service is just too much of a crap shoot.
Good luck!!