Tips on inspecting/buying a 40 year old car?

When I bought my Dart, it had been sitting for quite some time.
Some things I had to do almost right away,
Shocks, all 4.
Sway bar links, and I inspected and replaced anything that was rubber on the suspension.
If the brakes are touchy, I'm willing to bet that a rear set of shoes are hanging up.
Look at the frame, the torsion bar boxes and the leaf springs mounts. Both are areas that are prone to rust and rot.
The good news, alot of those cars had factory undercoating. The bad, just as many didn't. So if you look under there and see really old undercoating, don't be surprised.
There will be things that break and generally make you nuts. Don't sweat it, most are easily found if you ask.
One thing I did see, a tan distributor cap. It's either original or it hasn't been changed in a while. So I'd say a tune up is in short order.
The rusty booster and other parts, unless they're trailer queens, it's an affliction they all seem to get. Call it patina. Nothing a little steel wool and time wont fix.
$5K is about right, so you're not getting taken. If the car is what is advertised then I'd say you found a good car.