I checked cars.com...
...for S Class 1986-1991 cars within 150 miles of NYC (don't know your zip) just now.
21 are listed - 8 over $10k, the remainder under. Mileage range for the latter is 95-213k, where stated. Most are at ~120k. Median price looks to be about $6k - all flavors - 300SDL/300SE/420SEL/560SEL.
At 120k with a few $ into the engine you s/b fine. I would try to find the best body and interior - engines are cheap to fix, bodies are not.
These are true bargains if they have good records. No need to rush - lots and lots of these out there. You could always opt for a west coast car (better climate) and have it shipped, and still get in under $10k.