I M experience these cars only have major problems if worked on by 'experts'. Check for good service history, accident damage etc. Then check engine bay for ANY leaks of ANY fluid description ANY where. Get it on a hoist if possible.
Test every system for full function. Take it for a LONG road test without the radio on. Base your decisions on your findings. They are a complex vehicle but very satisfying to own and drive.
Good - Fast - Cheap: Pick any two.
E23 735i; E30 318i; E32 735iL (X 2 - 1 White and 1 Light Blue); E34 535i; E38 735iL; R50 Cooper; R55 Cooper Clubman.
E21 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i