Laying next to the vehicle so you can see the idler arm, engine off, have someone turn the steering wheel back and forth just until resistance is felt. If you see side-to-side play in the idler shaft(instead of rotation), the bushing should be replaced. It was easy on a 190E 2.3, but I have heard more difficult on the 300E.
I haven't replaced the damper yet on ours, but I suspect it would take some of the shock out of rough roads, and is cheap through Fastlane. Steering sector endplay can cause problems too, though, and this adjustment looks like a b_____ to do on the car.