info needed: BMW e12 ?

I used to have an E12 525 (carburettor model) 4-speed; very reliable car considering it had done over 110k miles. Rust-wise it depends on how the car has been treated/mistreated, but check sills, floors and boot in particular; floors suffer from shonky windscreen seals and blocked sunroof drains.
518 has an M10 (in BMW-speak) OHC four; has a timing chain but check for service history as they can take a lot of abuse. Earlier 520s shared engines with 2002s, later 520 has M20 OHC six with timing belt, so check for regular servicing and belt changes. Rough running on these engines is usually down to knackered carbs.
Pretty much anything can be slotted into the bay (check the blue car in the pics; it has a 427 small block Chevy lump), but if you're starting with a smaller engined car I'd suggest getting the brakes and diff from a 528i or if you're really lucky, an M535i.