Assuming the cosmetic appearance is good, that's a superb value! 81,000 is just breaking in an M103 engine. You can expect the engine to last 200,000 to 300,000 miles easily.
Some things to look out for though: head gasket problems -- if this has already been replaced then that makes the deal that much better. It's not a matter of if, but when you'll have to perform this maintenance. (~1k$)
Also, if the car has the original radiator (plastic neck), it will eventually become brittle and break. New radiator ~300$.
What options does it have on it? Sunroof? ASR? Heated seats? I think those are the only ones available for the '91. (MB Tex might've been, not sure)
If you keep the fluids fresh and maintain it well, you will enjoy years of enjoyment from this great car!
Also, any problems that might arise with the 300E can be solved on this site, either by posting or browsing thru the mountains of archive information.
'91 300E, 208k miles