They make replacement rudders for lots of sailboats, and the product is very good. Don't shop further, Fossfoam is the one.
With regard to the rudder dropping out because of one bolt, that's suspect. Most rudders have at least one more bearing inside the hull. You might be looking at this boat because it's a good price, but if an owner let that bolt corrode that much, I'd be worried. I'm on my fourth fixer-upper sailboat and it would probably be a good idea to keep looking. It's amazing what opportunities you can find.
I know of a Caliber 28 (not mine-- I have a C28 also) in Annapolis in great shape (all new sails and
rigging, with diesel
fuel polished) for about $16,000. Email me at marinewoodwork@gmail for more data.