I bought my 86 300E 2yrs ago and I'm satisfied with it. There are a few minor things wrong with it, but overall it's solid.
If their asking price is $2,100 I'd be a bit skeptical. If it's runs good mechanically, it's a steal. As with buying ANY used car, get a professional to inspect it. I suggest the MB dealers. This will give you a clear picture on the car's true condition.
Keep in mind this is a 17yo car so there will be quite a few things that may need replacing. But if the previous owner took care of the car, I don't think you'll run into problems that are out of the ordinary.