Alot happens in 30-40 years. The thing is to find out what has happend or not happened as the case may be for a particular boat. "Well Maintained" is pretty much advertising jargon.
You can break a sailboat down into basic systems. Not all of a "well maintained" boat will be 30-40 years old.
First, hull and deck.
Then mast spars and standing rigging
Then sails and running rigging
Followed by auxillary.
And last accessories.
Only one of these categories will for sure be 30-40 years old. Everything else could be somewhat newer. As far as I know, fiberglass has not been around long enough to know its finite lifetime, yet is has been around long enough to know what kinds of conditions, events and processes can cause it to be a write off.
As a general rule, if the auxilary is 30-40 years old and has not had a recent overhaul, then don't bid on that particular boat. A replacement engine can cost more that the market value of a good old boat.
Also, as a general rule, any electronics older that about 5 years should be discounted to no value. (If you have to haul the boat to replace an old depth sounder, it had no value at time of sale.)
Just make sure you are onboard when the survey is conducted and have the surveyor show and tell you what items he finds to be in need of repair
or replacement. And make sure the surveyor understands your quality requirements. After the survey, you can spreadsheet every thing you require to make it safe and enjoyable, and do your decision making from there.
Have fun shopping, and read all of those "which boat" threads. They are fun and interesting and full of opinions and good advice.