Take a look at the blue book for the oldest version of that same car and then do an adjustment down for age and mileage (and obsolesence).
Three years ago I bought a 1990 300E and the blue book was almost $7k.
At that time the car was 11+ years old.
Of course you'd be a dream customer for a dealer since they can almost never sell a car once it has reached 100k miles anyways so you've got bargaining room here.
Beware - eventually this car will leak oil (if it doesn't already). Oil leaks aren't themselves expensive to fix but the labor is tremendous unless you're willing to do this work yourself. See my posting today about this very point.
BTW - don't forget about A/C. The crossover to the new freon wasn't until 1994. You'll pay through the nose for the old stuff. Fortunately, you can sometimes get lucky like I did. A local shop was able to flush and re-charge my 1990 with the new freon (no new parts needed).