I talked to several AC shops and independent mechanics about this issue at the beginning of summer. The consensus was that there are at least two different compressors used in older Mercedes'. Neither is suited for the higher pressures created by using R-134a. My sources told me that the compressor would not last longer than two years after a conversion.
I can't recommend this procedure as long as there is any doubt about compressor life. Anyone on the forums who has done these conversions that has had one still working after two years should post their results so we can be more confident of the longevity of this type of program.
Also, I would strongly agree with Larry about evacuating the system, and even considering replacing the dryer and accumulator.
"We drive into the future using only our rearview mirror."
- Marshall McLuhan -
Northern California Wine Country...
"Turbos whistle, grapes wine..."