I believe that this is the engine that was used in the 6 cylinder gas models of the 123 sedan. This model was not sold very widely in the US, but widely sold in Europe. This engine may very well have been used in an S Class car as well.
If this is a European car, look it over closely for rust. If the car is in good shape, it sounds like a real FIND. I say that because I'm a manual transmission fanatic. I have two grey market Euro cars, and the only problem is that the insurance companies will shrug their shoulders when they look up the serial number. If you have the time, you might want to get the serial number of the car and call your insurance companies 800 number and ask them what you will run into insuring this car.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in