Oh don't worry on a MB the engines you will blow it apart if the chain breaks. The models that usualy bust the chains are the older V8's. The plastic guides get brittle and snap, really the chian doesn't fail the guides do.
But usually when they do snap, the valves will knock the piston with such force that the cam breaks and very often the valve becomes part of the piston. Usually happens on start up. I have seen pics of failures at higher rpm's, the results are quite amazing...very often a large chunk of the valve cover is blown off...
Sleep easy diesel owners, I cannot recall hearing of a timing chain just failing on a diesel. They are all inline engines with nice short chains. I have heard of more oil pump chains failing on the 617/602's actually.