From experience with my 300E -
Do a search on "AC" or "R134" and you will find advice from Benzmac and many others.
If I were you, I would fill it with R12.
I had a seized compressor. My decision was to replace the compressor, drier and its switches, and expansion valve and go to 134. Total cost of parts was about $600 from the PartsShop.
My mechanic says you must change the compressor because the oil is contaminated and you must change the drier and get the R134 switches. The exp valve is "calibrated" for R134. The majority felt that the condensor replacemant was overkill.
Falls Church VA
'86 300E 5 speed
'95 C220 (wife's car)
'98 Porsche Boxster
Past: '79 280E, '82 300D (18 yrs), '77 240D,