To me the inside knowledge about C&C's is key. Just where is the juncture of the keel with the stub...at the hull or does the stub end 6-12 inches below the hull. I looked at a boat where the latter was true, but there was still cracking at the juncture of the stub with the hull. I walked away, as it definitely indicated a weak design and, in that case, there was further damage aft indicating a severe grounding had occurred. Did you ask the owner if he made any insurance claims related to a grounding? I'd also walk around the dock...some of his dock mates might know too.