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FWIW, on my 280SL project, decided any increase in value wasn't worth the trouble of getting it working. I sold the authentic AC setup that came with it. One thing - you will need to find a regular alternator bracket. I am told they are NLA from Mercedes but there seem to be plenty of used ones around.
My question is whether A/C adds any value to this car?
So an AC might not add to the price of a car, but in some places it might add value.