Hi,
I just bought a 1991 300d 2.5 turbo. I would recomend reading the "Illustrated Mercedes-Benz Buyer Guide", by Frank Barrett. The book has a great deal of information. You may want to also read the past postings on this board. One thng I do recomend you MUST do is get the car looked at by someone who knows MBZ!!!! I know cars pretty well, I race SCCA, but when I took my cat to MBZ specialist, he was able to pick out about $1,600 worth of normal wera and tear items for a car with 165K on it. The seller deducted all of this. Some of these things were not really that bad, but Maryland has a VERY strict state inspection for registering a used car. It needed a timing chain + tensioner, transmission gasket, rear shocks, exhaust, tie rods, oil pressure sensor.
Needless to say I absolutely love the car. I have had many luxury cars from Lexus, to BMW to Jeep Grand Jerokee to Volvo 850 which were all bought new. This MBZ is of far superior quality even with 165K on it. The gas mileage is about 35 city 41 hwy.