I think the big expense on a/c's on MB's (i.e. 124's, 140's, 202's) is the evaporator, which is buried underneath the dash.
I've read it's about 15 hours of labor on a 124 to replace one, plus whatever the evaporator costs.
I've also read that it's better to use an aftermarket evaporator from a company called ACM, as their units are made from aluminum.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".