I'm assuming you are talking about the 89-92 years. Mileage will probably be in the 19-22 mpg on the highway; they had a lower final drive ratio than non-4matic E-class.
Problem area was the transfer case, which usually needed replacement by 100,000 or so. CV joint boots should be looked at for cracks. And the usual E-class /M103 issues - oil consumption (head gasket, valve guides/seals, front covers), air conditioning.