From what I have been able to read, the 300Es in the w124('86-95) seem to have adequate air conditioners, but that if something goes wrong with them they are labor intensive to get at, and if they are not well maintained, or have been run with low levels of refrigerant, or improperly converted from freon to the newer refrigerants, then expensive problems can occur. (correct me if I'm wrong on this, folks!)
I have an extremely minor leak in mine - the tech says it is not worth fixing until something major goes wrong and the system has to be evacuated and taken apart. He just checks it each time it is in for service for freon level. Certainly the prices I am seeing on the parts (compressor, evaporator core, etc.) are comparible or less expensive then the same type of parts on my '88 Acura Legend coupe.
If the car as low miles and excellent service records, looks like a well maintained cream puff, and has been OK'd by a good tech after a thorough inspection, I personally would go for it. All the positive features on these cars (w124 chassis) are described in the "cars" tab at the top of the page, where the solid construction, great handling, and high accident survivability by occupants have caused people to describe this model as the best used car choice of any make/model.
Good luck on the car search!
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E