Perhaps you already know this - while it is apart, replace all the vacuum actuators for the HVAC system. Mine was done by an "Indy" about 5 years ago for $2,100 - $2,200. His labor was only about $10/Hr. below the local dealer but he had about 25 years experience working exclusively on Mercedes products and had actually done more of these jobs than the dealer. The failure was indicated by the telltale oil leaking from the AC drain hoses. Placed a dye in the system just to double-check. While at it, replaced the heater core and expansion valve. Neither had signs of failure but didn't want to open the dash again for a future heater core problem, even if the failure rate is not very high. Additional cost was a little over $100 for the core. Still has the original compressor and has held full charge since the job. Cools real good! Don't convert, keep R12!
OliverB