Rust !
Most of the aircraft that you are talking about are getting on a bit and corrosion is becoming a real problem, the bits don't cost a lot but the labour costs to fit them are a real killer.
The places to look for these problems are around the steel fittings that are riveted to the alloy airframe, so inspect the windscreen posts, under the baggage bay at the rear spar attachments and step attachments, Stab attachments forward of the rear most frame (FWD of the bearing fittings) and lastly the flap ribbs forward of the flap spar for this you will need a boroscope (access via the lower drain holes).
These are basicly sound aircraft if looked after, it will give you a clue as to how the aircraft has been maintained if the owner has had Piper service bulletin 977 done. This corrosion prevention/detection SB is non mandatory and is quite cheap to do and if it has not been actioned it shows an owner who has skimped on the maintenance.