I would suggest you search on "A/C conversion". There is a huge amount of opinion on this. The consensus AS I UNDERSTAND IT is that the following MUST be replaced if you convert:
Compressor and clutch $425 - $1200
Drier and switches - $100
(You will need these even if you don't convert)
Expansion valve - $50
MOST recommend the replacement of the manifold pipe - $353 - $684
SOME (Benzmac) recommend the replacement of the condensor. - $250 - $350.
ALL PRICES ARE PARTS ONLY!
You will also need to flush the evaporator and condensor if you don't change it.
The benefit is that you will be environmentally responsible and won't have to pay $50 per pound for freon.
My advice - if you are going to keep the car for several years, do the upgrade.
Otherwise, buy the rebuilt Behr compressor/clutch for $400 from PartsShop, add the drier and switches and stick with R12.
WHATEVER YOU DO, DO NOT ALLOW THE COMPRESSOR TO SEIZE! WHEN THAT HAPPENS YOU RUN INTO A WHOLE ADDITIONAL LEVEL OF PROBLEMS BECAUSE YOUR SYSTEM WILL BE FULL OF TEFLON COATED GOO!
Falls Church VA
'86 300E 5 speed
'95 C220 (wife's car)
'98 Porsche Boxster
Past: '79 280E, '82 300D (18 yrs), '77 240D,