Soft spots in the deck of a seventies boat are not uncommon and are a cosmetic problem, not a structural problem. When evaluating this area and the ports a clean professional finish usually denotes good work. If the ports are secure, don't leak, and look well-finished; I would accept them. The rudder, of course, is of structural concern. It is not uncommon for delamination (port & starboard) cracks to appear in a old boat. The typical and best repair is to open the rudder; remove damaged core material; check for corrosion of the post and tangs. If the metals are sound, the rudder can be successfully rebuilt with fiberglass cloth tabbed over the original seam. Often, rudders rebuilt in this manner are stronger than what comes from the factory. I think a survey for purchase is always a good idea. 'take care and joy, Aythya crew