My most trusted engine tech of all is my brother. He specializes in motorcycle racing engines, however, and he is on the right coast whilst I'm on the left. Anyway, he does do car engines as well, and he mentioned that certain MB models are notorious for soft valve seals that wear out quickly. I haven't explored the issue yet because I don't have that problem. But I would most certainly check the valve seals before committing to something more major. From the clouds of smoke you describe, however, I suspect you will be doing more than that.
1995 E320 Wagon Moonstone Grey/Parchment 106K mi.
2007 E63 AMG Graphite/designo 75K mi