I had pretty much tyhe same problem though not quite the same on m '88 300E. Rough idle, would drop down to 500 or below and the car would surge a bit at low speeds when coming to a stop. Turned out to be a leak at the power brake vacuum booster.

A very knowledgeable guy suggested that, because of the surging, I probably had a bad booster and that I should disconnect the vacuum line and plug it. If the idle smoothed out then it was the booster. All I did was tighten the fitting at the booster and the problem went away.

Smooth idle, no surging, smooth, even acceleration. It's worth checking out the vaccuum booster.