I know a little about older VW diesels but don’t know how much of that translates to a Benz diesel but I’ll make a suggestion. Make sure that your glow plugs are getting power with either a voltmeter or a 12 volt lamp. Non TDI VW diesels need help starting from the glow plugs with a cold engine.
It might be risky to change the fuel filter at this time because that introduces the possibility of another reason for failure to start, that being air bubbles in the fuel line. A couple of ways to deal with this possibility are the use of a small hand operated vacuum pump and prefilling the new filter with diesel fuel. If the former owner did not change the filters and threw whatever he pleased into the tank, you may have no choice but to change the fuel filter(s).
There are plenty more things that might cause your non start problem. I suggest that you find a Benz diesel site or get what you can from the Technical thread at VWVortex when old VW diesels are discussed.
You should have ordinary diesel fuel in your tank to eliminate another potential source of a non start.
If the car has a manual transmission, you might want to get some experienced help to attempt a push start.