Good idea going for the warranty. One of the reasons my wife sold her 1998 E300Dt was this glow plug problem making her car run badly all the time. Very very annoying.
We had tons of other problems with the car, and have talked to a few W210 diesel owners that are dissapointed at the quality of the car.
She doesn't like the 'ol 190E near as much, but it runs like a top, day in and day out...
1998 C230 "Black Betty"