This is a residue of some sort that is burning off when you start. It is taking place all the time, but there is only enough of it to smoke when you start up and the plugs burn it off.

I am betting it is the valve seals. They are leaking oil past them when the engine is shut down and it is pooling on top of the pistons. When started it all burns up at once and comes out as white smoke.

You can remove the vac line to your brake booster and see if continues to smoke. You will then have manual brakes, but you know this going into it so you know the first stop will take a great deal more effort. If this stops the smoke then you will need to replace the booster since it is leaking inside.

The good thing about the valve seals on a Mercedes is that they can be replaced without removing the cylinder head. This takes a shop that knows what they are doing, but if someone is in the Mercedes repair business they should know how to do this. It is something best left to an expert, but you could do it yourself if you are VERY mechanical since it involves removing the camshaft (which is easy) and then replacing it (which takes forever to time and set valve clearances) but it can be done.

Another good thing is that these parts are not expensive. There may be a bit of labor involved, thought.