I would actually be more concerned with any significant damage to the rudder, which is way more sensitive to groundings & a lot easier to damage the rudder, the bearings or tube, then a lead keel. Rudders can be big bucks to fix, if the bearings or rudder tube get warped or bent. A new rudder itself can be thousands to replace. As a reference point, I paid $9,200 for my 1980 C 30 back in January. It was in decent running condition, with newer UK Halsey sails that are worth a small fortune to replace. My boat only has the lil 5411 diesel. An 85 should have the Universal 25, a much bigger engine. Most surveryors don't even run or do a compression test on the engine, as their not mechanics. This is where a diesel mechanic's inspection was more helpful to me. Make sure your surveyor includes a cost to cure analysis. This can give U a good negotiation tool w/ the seller.
When it comes to boats, a bargain is not always a bargain. But even with a well maintained boat, there are no guarantees. If the engine is a runner, & sails are good, then it may be worth the investment. I just recently went to look at a 84 C 30 in "good shape" that was a total lemon / money pit. Go with your gut instincts. Cheers, Jeremy