Hey nix, sorry for the delay but things are even more busy than usual
About the E38, as you know I owned 3 including my current 740d.
Things that go wrong are usually:
pixels in the dashboard (not so much on MY2000>)
powersteering hoses usually leak, 3-5 hr job
notorious 40-50 mph steering shudder, usually track- or controlarms , as you know from E34
Other things I had on my 1994 E38:
failing door lock actuator (lots of work)
returning cracking sound in driver door, can be temp fixed by adjusting the receiving hook of the door lock on B-pillar.
CD changer often fails
electric roof switches fail
headlights on pre-facelift are glass and easily broken , cost little money IIRC, I changed them for looks.
Try to find a car with the Brembo brakes, although most of them have these in Europe. They are alloy 4-piston and much better than the sliding single piston cast-iron. Recognisable by the flush surface whereas the cast iron look, well , cast