I did the 134a conversion on my wife's 89 300e. It works fine. The conversion was done this past Feb 2001. I purchased the $39 conversion kit from Wal-mart. It included the adapters, oil, and 134a.
I had spent BIG bucks on the AC about 2 years ago. A hose, ~$600, had a leak in it. So it was replaced and recharged with R12. That worked for about a year, then sure enough it started to blow warm air again.
I searched in the forum and saw that some people did the conversion without any compressor problems. So I figured I'd give it a try. Most of the R12 had leaked out so I didn't bother having the system evacuated. There was no pressure on the schauder <sp?> valve so I figured not much R12 was left.
I followed the instruction on the retro kit and installed the 134a and oil. If I remember correctly, I used 2 of the 3 cans of 134a. I used the supplied pressure gauge to make sure I did not over charge, remember to only charge to 80% of R12 capacity.
Got in and sure enough, cold air was filling the cabin. Now this was in Feb in North Carolina. The big test was how it would perform in the hot and humid summer.
Well even in 100 F 80% humidity, the AC works like a charm. My wife is very happy. Especially since it only cost $39 and not $1000+. I check the pressure monthly and have NOT had to add any R134. I don't know if this is due to the larger molecule or if the oil sealed the leak, but I am very satisfied with the conversion.
It DOES NOT cool as well as the R12 did when it was working or as well as my 134a in my 1992 400e, but we are talking maybe 4 or 5 degrees difference, and THAT is on the hottest days.
As other members have pointed out, this is a well covered subject. Do a search and look over the data, that's what I did. If you do decide to do it, then good luck and post your results.