With reference to your consideration of buying an older boat... I presume
though you did not say, you are looking at a fiberglass boat rather than a
wooden boat. If you are looking at a wood boat, all bets off as far as my
comments. People will spend a fortune to buy a boat like they had as a kid
and sink thousands in it to recover their childhood memories.

Older fiberglass boats had wood frames, wood stringers, wood keel, wooden
stem, and a wood transom. These were all encased in fiberglass. The floor
was typically common plywood and was glassed in place. but often there was
no glass or resin on the underside of the floor. As for the floor, a boat
of the age you're talking about it would be common to find the floor rotted.
With water in the bilge, the bare underside of the floor soaks up the
moisture and finally rots. Also when the seats are secured to the floor
this creates an intrusion through the fiberglass floor and becomes a place
to let water into the upper side of the floor.
As hardware is secured to the transom, such as handles, tow eyes, I/O
drives, trim tabs speedometer pickups, depth finder transducers, etc these
holes through fiberglassed transom then allows water in, so a soft transom
is common.

As for the frames, keel, stem, and stringers, it is all a matter of how well
the manufacturer encased these items. It is common for water to find it's
way in, but no way out, hence rot.

The quickest and easiest test for rot is to walk all over the floor and even
jump a bit to test the floor's strength. If the boat has an I/O drive take
your finger and press in around the transom mount for the I/O and see if it
seems soft. If it is an outboard, let the engine drop somewhat abruptly and
see how well the transom supports the engine. Or if on a trailer, put the
engine in the down/run position and put some pressure on the lower unit with
your foot to see how the transom reacts.

I can make these comments because as a partner in a boat repair business, we
have ended up with boats that people didn't want to go to the expense of
repairing all the rot. Often they will leave the boat/motor and trailer to
get rid of them when they realize the cost to repair it all. The cost vary
but it costs around $100 to dispose of a boat in a land fill. By the time
we pull the engine and out drive if it is an I/O it's a crap shoot as to
whether the engine and out drive are useable or can be resold. We try to
run them before we pull them from the boat. But if they don't readily start
we don't invest excessive time in them, because even if they do run, we may
never find a buyer for older engines. If they run we keep then and hope for
a sale. If they don't run we try to salvage useable parts to resell or sell
them to someone who is looking for a parts engine he can rebuild. In
summary, buy the time we have invested the time I just described, plus the
land fill fee, we MAY break even by selling the trailer if it is any count
at all. That doesn't always go real well either, most people expect to pay
$75 for a tandem trailer with fresh paint and new tires!

Bottom line answer to your question... Look an older boat over very
carefully or you could be buying an expensive restoration project boat. If
it has the rot I described and you are inclined, you can restore the boat
yourself, but that can be a big and expensive effort. And you'll loose a lot
of nights sleep dealing with the itch from grinding out the old fiberglass
to install the new components.

Though this was rather detailed, I hope it answers your questions.

Don Dando