Good luck in your quest.
Rust is important including as you mention investment, appreciation, re-sale value. Later 240s have better anti rust protection. Other people will be able to advise about that and about where to look for rust. You mention the estate....light rust below the tailgate behind the rear bumper indicates rust out of sight and not cheap to fix.
If he learns mechanics on a 240, that will impart good fundamental skills. The design is old now. Does he want to learn newer design mechanics instead?
Few garages know the wrinkles of working on 240s now.
There's a lot of helpful knowledge in this forum. But some threads go on for ever before hopefully reaching a result. Sometimes the right people aren't around. Sometimes methodical people with knowledge make wrong assumptions.
Some spares can take time to obtain.
The k jet by the way has an inlet manifold designed to stand a mug of tea or coffee on, and a reputation for reliability, and avoids the diagnosis problems of jetronic.
My jetronic estate works for me.
Brakes issues can be elusive.
Start by following the very specific 240 bleeding instructions.
Before doing anything significant to a 240, search this forum, and ASK.
Good luck.