My '91 350SD had 215k on the original, somewhat neglected engine and still ran fine. But, for the most part, the engines are junk and for a prospective owner, I will tell them all day long not to buy the car. It simply isn't worth the risk of a $10,000 engine.

For current owners, all you can do is keep them running and hope for the best. And 1qt every 1500 miles is within spec but certianly not normal (I think MB calls abnormal consumption at 1qt every 800mi). My 300SDL with 300k on the clock was using zero oil at 4,000 mile intervals.