It's the aftermarket water pump. Go get a reman pump from the dealer - last I checked it was priced at $25. Compare the impellers between the MB pump and the one you got. I bet there's a dramatic difference. The factory pump has a cast impeller with very tight clearances to the water pump housing. After market pumps I've seen use a smaller cast impeller, sometimes with fewer vanes. The really bad ones use a stamped sheet metal impeller. These are guaranteed to add 10C to your coolant temperature

With a little practice you can get the water pump out of an old 300D in 20 minutes...

It's normal for coolant temperature to run as high as 100C if you're working the car hard on the highway, a/c blasting, and hot weather. However, it should stay near 90C in gentle stop & go traffic.