Be sure to check that your new compressor has metric threads in the mounting holes.
The pressure switch in the reciever drier should be replaced with the updated version. The new one has a cut out pressure side that will cut off the compressor if the high side pressure gets to high (approx. 450psi).
If you have an air compressor it is a good idea to take an air blower and clean out the fins of the radiator and condensor to ensure good air flow. I have seen this simple thing lower the high side pressure 50psi. Make sure the aux. fans are in good working order as they are extremely important especially when using 134a.
Remember when charging your new a/c system with 134a that the charge amount is only 85% of what the system held of R-12. Over charging the system causes reduced cooling and possible damage to the new compressor.
Hope it goes well for you and you have frost on the vents when you are done.
14 year MB tech
7 year shop foreman @ independent MB/Volvo shop