My 1982 300D does the same thing as far as the light blue smoke at startup. Not much at idle.
If you are not used to diesel cars, then the Mercedes does make a sound like clanking at idle that is louder than what you might be used to.
It's hard to answer your question without knowing just how much smoke and clanking there is - but I plan to drive mine just like it is for a long time unless it gets MUCH worse.
Another possible cause: valve stem seals
308,000 miles here so far.