I have had 4 mb's in the last year with inoperable ac, every single one took a freon charge and started operating properly. 2x the previous owners gave me the same story that mechanics had said the system needed to be replaced at a cost of 1200-2000.

I would buy a can of refrigerant and fill it. Best $25 you can spend. Likely a small leak that over time has left the system low and keeping the compressor from engaging. You will still have a leak but if it worked for 12 years you will likely get several years out of a charge.

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1981 300TD
2005 C55
2004 E500
1999 ML430
1992 400E
1998 C43
2001 SL500
1988 300SEL
1986 190E 16V
1991 190E 2.6