In my mostly hearsay experience of the 35 years I have had my J22, I have heard that the bulkhead panel where the chain plate attachment of the mast stays below deck and the keel attachment are the major areas for concern. I have never personally bought a J22 since buying mine new in 1988. Knocking with your knuckles all over the panels should reveal no “soft” areas and the area of the keel attachment inside the cabin should remain firm and have no areas of “softness”. I have been told the panels have balsa cores and, if the chain plate area on the deck has not been properly caulked and water sealed, the balsa will rot and lead to eventual stay failure. Same applies to keel if long term water exposure in keel attachment area inside cabin has occurred. Have been told that can detect by holding keel at base and moving back and forth laterally while other observer is inside cockpit looking at keel attachment for movement and cracks.
If have local J22 owners, might want to have them look at boat with you before buying?
Good luck. Love this boat. Guess that’s why never had to buy another.