George Murphy sells rebuilt servos and amplifiers, and vacuum diaphragms. IIRC, a rebuilt amp from him is around $100.
Used working ones are $30 shipped on ebay. Search "Mercedes climate amplifier" to find them.
They look like this.
I had to deal with fixing the servo climate control system, and it completely works (it's nice!) so if you need help feel free to ask me. There is a learning curve on ACC work, particularly the older version.