Maybe one of the professional MB Techs have some other reasons to stay away from this model, but the one I'm aware of, can be easily remedied. That is, '86 and '87 diesels came with a trap oxidizer. It is a huge catalytic converter sort of contraption above the exhaust manifold on the passenger side. If it is still in place, Mercedes will replace the entire exhaust system to remove it for free.
The other items should be probably be negotiated by determining what a dealer would charge to repair them all, get that much off the price, then take care of them yourself.
A friend of mine bought an '87 diesel with almost 300,000 miles. He has driven it for several years with no problems. He was a diesel truck technician for many years and went to work at an MB dealership as a tech AFTER buying the car. His car has had very frequent oil changes and is in incredibly good shape still.
$7,500 sounds like alot for this car. Get them down on the price.
'01 C Class, Six Speed
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in