I have the same car, a 1995 E300d. I paid $4800 for it (retrospect, a little steep, but whatever). The suspension will need some love, but its usually a 1-time thing for an owner.
I did have an injection pump die on me - the replacement from benz is $5k. (Not a typo) but there are good used ones out there for $250-500.00
Motor isn't tough to work on, just need patience, a surgical cleanliness propensity, and some good wrenches. Its a mechanically injected inline 6. About as simple as it gets on a diesel.
Agree on harness - had to do that, not rocket science but annoying.
Injectors aren't tough, they do need special tools (I have, and rent them out, just sayin..), the glow plugs can be a PITA if they get comfy in the head.
Gotta use Liqui-Moly Super Diesel additive in them -
turned me on to this stuff, and its gold. Amazon has it by the case. bottle every other tank.
Diesel cars are for highway warriors. I drive a LOT (have rental properties around central AZ) and this car, for 2015, has cost me maybe $400 in maintenance. 2014 was more, for sure, but once the big stuff is done, its one of the best vehicles I've ever had. They don't like to sit, however, so if its a once-a-week driver, you're gonna have some problems.
Offer them $3400 cash and just know its 23 years old, and not perfect.
The next nut I have to crack is a new driver's seat...