You may want to do a little background work to establish if this is the original engine or a replacement at some point. Or perhaps engine has been rebuilt? Anyways before really starting to spend money would do a compression check and leakdown test after valve adjustments and glow circuit properly check out and if situation is not improved. Then with good or reasonable numbers you may start looking for things like slow starter, weak battery, air in fuel or anyplace you want to look. But if numbers turn out bad or marginal you pretty well have your answer unfortunatly. Also would cause no harm to check condition of timing chain and pump timing as perhaps have not been done in a long long while. This of course eliminates possibility that chain is about ready to go as well. Of course you might have the service records on this car and that would be most helpful I would think. One post suggested just using car above it's normal starting temperatures and thats pretty positive as well if engine turns out to be quite weak. Edit: previous post went up when I was compiling mine. Like his ideal of checking each glow plug by current draw and will have to think about it. Problem is to get an ampmeter with large enough range to be practical. War surplus? You can learn to get by with a block heater in a lot of instances but again not in all. I know I have diesel cars that I use in winter months that could have their cold start temperatures lowered with some effort but just get by using the block heaters instead. This also eases the cold starts and helps prolong glow plugs, starter, and other components lifespan. This area is one of the most commonly brought up on mercedesshop site. Probably a lot of helpful suggestions in search format. Always wondered how the german army got these engines to start on the eastern front in russia during the winter. Finally decided they probably never turned them off. Perhaps they were not used on that front but the russian tanks had diesel engines so it was possible to start and run diesels way back then at extremes of low temperatures. The germans appeared to spend the design effort on their gas designs as two of my early post war mercedes gas cars always were capable of starting at extremes of cold when my american cars were not interested. Each required a large capacity battery as they were very poor starters in comparison when block was at operating temperature though. What I really am getting at is that the germans could have designed a better cold start system for these early diesels in my opinion and it's a shame they did not.