I would try for a 90 or newer 300E or 300D 2.5D T. The 300E won't get your fuel mileage requirements, but will be easier to find.
The interiors were updated at that year and the body trim also. More radio and the electronics get some help with some self-diagnsotics.
My suggestion after that is to buy miles and condition. Don't go for the cheaper car. Try for 100k or less, especially with your amount of driving. If values in your area place these models out of range go back a model instead of buying high mileage. There were no diesels in the "E" body between 87 and 90. I don't particularly like the 603 diesel, but thats personal.
[Edited by stevebfl on 12-24-2000 at 11:24 AM]
Bosch Master, ASE Master, L1
33 years MB technician