Hard shifts and lack of power could be relatively easy fixes. Cosmetically, the car may be really great. That doesn't mean it's been well maintained under the hood. For $7k, I would definitely want service records and you should have it thoroughly checked out by a good Benz mechanic. Even if everything checks out, $7k is a little steep for an '82 with 200k.

For slow starts you may just need to clean the banjo. A valve adjustment wouldn't hurt either. My pedal vibrates at high speed, too. I know I need motor mounts, so that could be it. Hard shifts are probably due to a vacuum leak. Search around this site. You can plug a few lines to take care of it.

Not sure about your inability to pass on the highway. You may want to have the turbo checked out.

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1983 MB 300SD Turbo Diesel