Ah yes, the old "rough idle is normal for this model" reply. Well, it is very common (not quite as common as being told that the condition is normal), but you'd certainly hope that on a car like that it wouldn't be "normal".
I had a hesitation problem accelerating off idle with a 380SL (not identical but similar injection system). The car was also running with an extremely rich mixture. The warm-up regulator was replaced with no effect. It turned out to be the oxygen sensor. When they get plugged up or fail, they incorrectly sense a weak mixture send a signal to the fuel injection system which causes the fuel system to compensate for a weak mixture. This can cause a number of problems, including lousy fuel economy, hesitation off the mark, and so on. No guarantee that that's your problem, but you could see how the car runs with the sensor disconnected (when the fuel system apparently defaults to a safe setting).