Seriously, I would be very carefully buying one of these with any rust, it can be very expensive to fix. Consider looking at cars from other parts of the country.

and found it interesting that it goes through a long list of both air/fuel and mechanical things to check... and most of the suggessions in this post were included, but...

I did not see "EGR" valve included. I say this because I just solved my poor power problem by disconnecting my EGR on the '80 300D. Granted you are dealing with a turbo model and I have no experience there [yet].

If I were you I would check to see if your your EGR closes fully when the engine is shut down. Hopefully you can do this merely by looking down the throat of the intake. If you have a vacuum pump, then you can pump up the EGR line and see if it is opening and closing. It is my understanding that EGRs on diesels are controlled to open fully at low RPM throttle so this would mean it's not likely to be your problem... but I would check it out. I would even temporarily disconnect it and plug the vacuum line with a golf tee... just see what difference it makes when you test drive it. This takes all of 5 minutes ! If you have noticeably more power, then your work begins to diagose and solve whatever the problem is... possibly a EGR that clogged up with crud!? My independent M-B mechanic see a lot of this on mal-maintained cars!

Just my $0.02!
Sam