One thing you need to keep in mind when starting a diesel in the cold, is that you need to keep turning it with the starter for a long time. It's not like a gasser where you turn it over for a while, stop and then do it again. If you read the owners manual, you'll see that MB expects you to keep turning it over to build up heat in the engine. There are also different techniques with the accelerator dependent on the temperature. No accelerator when warm. A few pumps below 32 degrees and hold it to the floor in really cold weather until a few cylinders start firing. Keep the starter engaged with the few cylinders firing until it warms up enough to fire on a few more cylinders and it can run on its own.
I got mine started at 35 below this past weekend but it wouldn't continue running because the fuel had gelled.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13