I have a 1992 300E. The system was converted to R134a by the prior owner in 1997 when they had the evaporator replaced.

Last year I replaced the compressor, expansion valve and reciever/dryer and charged with R134a. The system works fine. I'm not sure but I recall the center vent temps would go down to about 40 degrees with ambient temps in the high 90 degrees. I live on the coast in South Carolina and it's very humid here. So far so good. I guess I'm lucky my system works great for now.