I had mine replaced last year at the dealer. The dash
has to be pulled and removed to get to the evaporator
and other components. You will probably have to replace
other components (or at least ones prone to failure) while
the dash is out.
About 1/3 of the cost is material. There is a lot of labor.
Unless your mechanic has done a number of these, I would
have the dealer do it. There is a lot of work to it. Chances
are, they have done this several times.
If you persue something with a dealer, ask about who does
the work and their experience, etc. They should also be
able to provide you with a quotation and a basic list of
parts so you have some idea of what is going in to it.
1993 300SD (286k + miles)