I have converted hundreds of cars to 134, maybe a thousand. Probably a hundred 300Es. BUT I haven't done a pre 1990 300E in almost 2 years. I do not do them if they have the 10p15c or 10pa15c compressors. This is writen on the back head of the compressor. These small compressors do not handle the extra load well; they didn't do great in R12 and they ABSOLUTELY will not do better in R134a. There is an absolute 5% decrease in thermal efficiency with R134a.
If you wish to convert (DON"T do it) fix your system first in R12 till it is cold enough to suit you and then make your decision.
Bosch Master, ASE Master, L1
33 years MB technician