All of the above replies have good information.
However, as I understand it the car is still in the hands of the mechanic. You need to give him guidance as to what you want fixed.
All of the issues are vacuum related and the worst case is you need a new vacuum pump. I just replaced mine on a 1983 300D and the part was about $200 new. I would expect about that much more for labor. This could be a DIY if you can safely get it home and if you plan to drive this car you will need to learn how to fix most of the problems.

You may be stuck with this mechanic for this repair as your car is not very driveable as is. If you tell him to fix all the vacuum problems, it can become very expensive labor wise to trace down all the vacuum leaks. I'd suggest you tell him to just get you back to safe & driveable and to block off the other vacuum users (door lock, climate control, etc.). The system is designed to give priority vacuum to the brake booster.

Then use this forum and the suggestions above to start chasing the vacuum and other problems on your own. I think most of us have found working on these diesels to be rewarding and therapeutic.

Welcome to the forum. I assume you are a student - college?
Myself, I'm a university professor of chemistry and physics. Had by 300D since new in 1983.

Charles