The valve adjustment would ensure that the valves can open far enough to let in enough fuel or to ensure that the valves don't stay open during the compression stroke. If they don't open far enough, you should hear a lot of clacking noise in the valve cover.
As far as rebuilding the top of the engine, have you ever done anything like this? Can you spare the time for the project? You would have to disconnect a lot of stuff, label it and then put it all back. The reason it costs $2000 is the labor charge. The good news is that the stuff usually comes off quite fast. Putting it back together is a different story. Can you check with a local technical college? They may do jobs like this, they may even be happy to see a Mercedes.
Good luck and keep in touch.