Any 20-year-old boat will need a thorough survey and possibly a major refit. At my time of life, I wouldn't want to be faced with a three-year project, so I'd look for a boat that had seen some recent upgrades in the key departments.
Hull: If there's any water in the bilge, I'd like to know where it came from, and I'd also check the integrity of the through-hulls and associated plumbing and eliminate any systems that weren't mission critical. I'd also want to know the osmosis history. And if the boat has an external keel, I'd like to see a report of when the keel was last dropped. **
Rudder:** Fiberglass rudders are usually built around stainless-steel backbones, and they often end up leaking. We all know what happens to stainless steel in salt water, so I might want to do exploratory surgery.
Rig: Stainless steel suffers from all manner of ailments as it ages, including fatigue. Standing rigging more than 10 years old probably should be replaced. Chainplates should definitely be pulled, inspected, and replaced if there's any suggestion of crevice corrosion. Aluminum masts tend to last longer than their rigging (if the rigging doesn't fail first) but are prone to corrosion where hardware and fasteners have been installed without insulation. The mast heel and step can be damaged by water in the bilge.
Machinery: The engine needs to be sound, but I don't need a generator. I'd rather carry the weight in books.
Tanks: Boats tend to outlive their tanks, so this would be a topic for considerable research at survey time. What are the tanks made of? Have they been replaced? If not, what kind of project might that be?
Electrical: I'd try to strip the systems to the essentials.
Sails: It's a sailing boat, so the condition and newness of the sails would be of great concern to me.
Everything else: I could add a whole lot of stuff to this list, but it could get very long. The bottom line is this: If the boat is sound and generally functioning, I'm good to go.