My experience with such old cars, where their age is starting to make full restoration advisable:

• mechanically reliable and simple to fix (and critical parts are usually available, quality varies)
• everything electrical is shit (and old), you need to go through everything to check the connections, completely rotten through ground wires aren’t necessarily visible and will cause endless pain until you find them
• the rust-proofing wasn’t great but it was there so usually if they’re still alive by now they just need the usual spots where those models rust checked before purchase and/or crash damage thoroughly checked
• crash safety is nonexistent, ABS and ESP aren’t that useful if you learned how to drive without, if you rely on ESP, better learn not to
• the fastest affordable cars from the 1980s are slower than family cars from the 2020s, still feels like you’re going way faster
• if you buy something predating catalytic converters in your country, be prepared for it to smell atrocious
• carburettors that haven’t been tampered with and aren’t leaking don’t need to be touched unless they malfunction, if they malfunction they usually only need a good scrubbing with diesel then a wash in petrol, replacement of extremely dead gaskets, and the jets usually need a clean, the tiny drill bits for carburettor jets can be used for cleaning if you use the size just below the marked size. Ultrasonic cleaners aren’t necessary, replacing all gaskets usually means it’s going to leak more than before.
• EFI was shit until the 1990s, avoid it if you can