Victor, it seems that your mechanic is b.s. you.
I mean if he only had to change couple o-rings for the small leaks, why does the system is clogged now?
If I were you I would takje it someehre else or try to troubleshoot myself.
The last thing you want to do, as LarryBible said, is to change your evaporator.
I have not done it for any benz, but I did ot on a Nissan (very difficult).

good luck