Typically the climate control unit or the evaporator are the A/C problems that happen with any frequency on the W124 models. The push button climate control unit is expensive...about $300.
The evaporator is even more expensive...the entire dash must be removed to gain access. If you farm out the work, it could run around $2000-$2500!
My evaporator died at 85K, and was fixed under the aftermarket warranty I purchased. So far, no problems with the climate control unit.
BTW, the tranny is also a robust unit. There are some that believe that tranny fluid should be changed around 100K miles while others say MB says it doesn't require fluid changes. Either way, it shouldn't call itself to attention unless it was severely abused.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car