I have no quarrel with MB's FIVE cylinder diesels, but in this case, I would prefer a '90-'91 300SE over the '86 diesel simply because all accessories and ancillary equipment on the SE would be several years newer. I also echo concerns about living with a diesel in very cold weather, particularly if the engine is a little worn and compression isn't quite up to snuff (making it harder to start). You may wish to search the diesel section on this site for further elaboration on this point. Another (minor) issue in this self-service era: In terms of aroma, no one has ever confused diesel fuel with a bouquet of roses. Splash a little on your loafers and track it into the vehicle and you'll know what I mean. Arguments about differences in resale value, while probably accurate, aren't overly persuasive given the relatively low value of either vehicle.
Have any serious candidate professionally evaluated, then buy it if you like it.