I have bought some used planes that were lots of fun- for a while. The longer I have them, the more defects I notice. Most of th defects are minor cosmetic issues, like monocote that starts coming up in areas of the fuselage that are subjected exhaust or fuel. When I tack down monocote in tough areas of the fuselage, I always run a bead of CA along the seam; lasts longer.
Another common problem: firewalls and fuel tank compartments not adquately fuelproofed. With this defect, things look good for a while, but if any fuel or exhaust contaminates the wood, after some time the wood gets mushy.

Bottom line: don't pay too much for someone else's airplane!