Not sure on 6 speed thing, im sure sir_dave will be along soon and be able to help. Mines only a 5, and i find a 6th gear would help on motorways.
Biggest problem ive found with the E36's is electrics. If he goes to look make sure he tests EVERYTHING, opens and closes the boot and checks state of wiring on left hand side of boot strut, and then checks all electrics again (rear lights in particular). They can be very fragile as the loom at the rear wears down over time, wires split and short and water gets in and is a pickle to fix and keep fixed. Many will have opened, resolded and repaired with tape to insulate it. This is pretty normal.
The tyres cost a fortune at the back (as is expected of what was once a very expensive car). Engine mounts/ mounts in general are worth a look too, drive it with radio off and find some bumps, soon hear if its knocking around.
Coming from a clio, the noise the straight 6 makes is fantastic! Most will think you've spent lots of money on it, they've aged well.
Mine gets me lots of comments, although i wish now id got a blue/ black/ silver one.