Take a close look at the block coolant drain plug on the right side of the engine to make sure it isn't leaking. You may want to put a wrench on it and verify that it's tight.

Unfortunately, from your various pictures it appears that the head gasket is leaking. Our daughter's 1991 300CE's engine had a similar appearance when the head gasket was shot. After a local indie replaced the head gasket the engine was bone-dry. As I recall the bill for parts and labor was $2,500 ... most of the cost being labor. An experienced DIY'er can do the job very inexpensively with the proper tools. Best to use a lift for pulling the head.

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Fred Hoelzle