Probably the best thing for a prospective legacy Piper Cherokee (and derivatives) purchaser to do would be to contact Piper, and ask for a list of parts available - in stock - for that model. The PA-28R-180, 28-161, and Seneca II have each been a real problem for my experience. The only thing that got the Seneca back in the air is that Piper did provide the information on the material that the spar carry through was made of, so "made parts" could be fabricated as replacements. This cost more than $10,000 by the time it was approved.
It's just a buyer beware situation. If the Cherokee series is being supported, I could not be happier, they're great planes, as long as you can keep them flying!