Hmmmm. Well, I always like to try the easy and cheap things first.

- Pop the top off the air cleaner and see if it is clogged. Perhaps you can idle it for a little while this way to see if the smoke is reduced.

- Try some fuel injector cleaner. The most effective thing is diesel purge at full strength. A fairly effective thing is a full bottle of Red Line DFC.

- I don't think this is it, but if the valves haven't been adjusted in awhile, it is probably time to check on them. And also whether or not there is any timing chain stretch. (a search here on these topics will find a lot of information.)

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So, the first two are really easy. The third is kinda hard in cold weather, and I'm not sure it would help - although it is something you need to do routinely on the car. Mercedes says every 15,000 miles - others here would say 20-30K.

I don't fully understand the fuel injection on diesels. But it seems to me if too much fuel were getting injected, it would simply speed up, not smoke. So for some reason you are not burning fuel correctly. This could be an air restriction - or maybe some kind of dirty injector problem.

Low compression comes to mind, but that's not consistent with good power otherwise and good cold starting.

Let us know what you find and maybe we'll think of other things to check.

Ken300D