[RS]

I think you are confusing "some" for "most" in your conclusions. My experience looking at 30+ boats from the 70s is that work on transoms is common. Often there are soft spots on the floors too. Rarely would it need the rest of the things you've mentioned, though many may elect to get guages and consoles. I have not run into any personally that needed stringers or new liners. Not to say that "some" don't go through that, but it's certainly not "most."

Also, $8 an hour for glass work? Not anywhere in New England, try $30 to $60.

FWIW, I have under $20K into my 20 which included a 1.5 yr old Honda with 50 hours, 6 month old 5000# aluminum trailer, new transom, new gelcoat on hull and gunwale cap and numerous minor glass repairs inside, new flush mount hardware all around and more.
All I did was rewire the boat and installed the flush hardware. I think is was worth it.