Check everything you normally would with any E34, then look for recent (big) bills for the SLS if its fitted, valve adjustments every 20-30k on the 3.6, battery drains, failing blower motors, the air con is unlikely to work, and then take it for a test drive. At this age i'd be looking for evidence of bills for rust treatment, MAF, Suspension, Bushes, Cooling system, regular valve adjustments, a clutch and use of the correct oil in services.
They are fantastic cars
I just read that you want one that won't have to be touched for a couple of years... I'd say even a good condition one, that is quite unlikely. Also i wouldn't worry about a sub 100k example, find one that's been well cared for, with comprehensive history.