Argh!

Do you think they affect flapped
gliders any differently than standard-class gliders?

NO – They are all built with the same techniques.

seeing aging gelcoat,

The nicer the finish, the more $$ you'll have to pay.

surface corrosion on metal bits

This should be to a minimum, but realize that any thing more than a
few days out of the factory will have some chipped paint.

worn-out cockpit interiors,

if it’s not new, the interior will be scuffed. Parachute buckles
make all kinds of marks. An interior is easy to fix, it’s purely
cosmetic and does not affect the performance of the sailplane.

You have missed the most important factor in the equation. That’s
the pilots ability to make the sailplane perform. I will guarantee
you that you will not notice a 10% difference in a “old” sailplane vs
the exact same model factory new. Your piloting skills will mask any
difference. One extra turn at the top of the thermal, a slightly
poor choice of track, flying to slow or to fast at any given moment
or even taking to many thermals will affect your ultimate XC ability
by 50% or more. Relax on the details, any of the models that you
have considered are good. The entire package is more important than
the specifics. A functional trailer, good working instruments that
you understand and a dependable electrical system in any sailplane you
fly will do.

The point is to get out and fly. Last weekend at Ephrata is a good
example. Even if you didn’t bring your own ship out, the soaring was
exceptional. 3 club ships were tied down and unused all weekend.
What a waste. I flew almost 12 hours in 2 days. Saturday had nice
thermals with wave over the Cu to 18,000 feet. Sunday’s cloudbase
was around 12,000 feet with almost everybody doing 500 km and some
well over 600 km. A low time pilot is always the weak point in the
equation. Maybe you should spend less money on the fiberglass and
more on your training. Buy a ride with Karl Striedeck at any of the
contest he flies. Doug Jacobs also offers rides. Gavin Wills gives
great XC instruction in the west. There are many choices and they
are all good.