The part of the A/C that is in the "heater box" is the evaporator, I presume. The only 126 cars we have replaced evaps on are the 90-91 model years; never on anything older. I would get a second opinion.
I also would check the expansion valve which also is inside the dash (but not inside the box). Unless the tech is familiar with MB he may think both pieces are inside which is not the case. Expansion valve leaks are common.