You should have a sticker on the front engine bay cross member that tells you how many lbs of R12 or R134a your system uses.
I believe all W124's after 1990 are already R134a.
If you have a W124 with R12, keep it, unless you're will to add dual aux. fans and upgrade to the R134a condenser as well.
Although I did the conversion correctly and COMPLETELY (dual aux. fans & R134a condenser, et al.) it still doesn't get as cold as R12.
R12 is actually getting cheaper.
1988 360TE AMG