Those of you that have tried to convert to R134 and are getting bad results haven't done your homework. I had mine converted by a A/C guy with 40 years experience. Works perfectly. In fact better than it ever has w/R12 (granted the R12 system probably wasn't in the best shape).

1) Best site for this discussion is http://www.aircondition.com.
2) True: R134 takes about 80% to 90% of the original charge. Most A/C people are just finding this out. You have to start at 80% and watch the temp and gauges while filling.
3) Not one of you has discussed a leak detector. The dye is not as accurate as an electronic sniffer. The system MUST first be leak checked. If the system is low on R12, there is a reason and IT HAS A LEAK. Most of the early 80's MB are at an age where the compressor or hoses are dried out and leaking.
4) The ND compressors lend themselves to rebuilding. The R4 rebuild results are generally poor and don't last as long. Get a new R4, check around for prices. NEW R4 compressors aren't that much more than a GOOD rebuild.
5) If the system hasn't reached the point where the compressor is starting to disintegrate (black death) you don't need to replace the expansion valve when converting to R134. You DO need to replace the receiver dryer. Get good German hoses if any leaks are found. Don't let your A/C guy try to "make up" hoses from standard parts. If he can do this, it is certainly a sign he knows what he is doing, but between the labor and the hose material, the costs are about the same and the Euro hoses look MUCH better.
6) The condensor coil for the R134 goes across the car rather than up and down for the R12 OR vice versa, I can't remember but that is the difference. I was told there is no aftermarket R134 condensor for these older MB's. New cars have the R134 direction. So you will have to stick to the R12 style condensor. It doesn't need replacing unless it leaks. Same for the evaporator coil. Check condensor coil for bugs etc.
7) The system needs flushing out prior to the conversion. Special fluid is used to clean and flush.
8) My system conversion took all day by a pro. New compressor, 4 new hoses, new receiver dryer. Didn't need 24 hours of vacuum.
9) In closing: get the system leak checked with the electronic leak detector. Skip the dye. Replace the leaking parts. Flush, change Receiver/dryer, vacuum, replace w/R134 oil, charge system with gauges and start low on the # of lbs. I agree with Larrym if the hoses are OK, replace the hose seals anyway. Find a good A/C guy. I'm in Los Angeles so this wasn't hard.