If the air filter was really filthy it sounds like the past owners possibly did nothing in the way of real maintenance. A lot of people also feel a diesel requires no maintenance. They just drive them till they stop or lose most of their power.

The 616 240d and 617 300d require things like at least checking the valve clearances once in awhile. Every 15k was the mercedes recommended interval. . Yet some have not been done for at least a hundred thousand if not more miles in my opinion. The clearances tend to tighten up with time and milage. This can impact the ability to start. I would start out by at least taking a feeler gauge and making sure there is some gap clearance on each valve. This does not mean you have to actually adjust clearances right now unless you find one or more valves with no clearance left. . If this turns out to be the case just adjust those tight ones for now. The approximate clearances are in the archives.

You obviously have to replace the primer pump.

Also trying to jump start one of these cars with a flat battery and a set of booster cables may not get enough current to the starter to initiate combustion. The engine is just not turning fast enough. So charge and check the existing battery. They will test it for free at many places. I still have a couple of 240ds. One is a nineteen seventy nine as well.

His original problem was probably fuel system related. Pay attention to the tank filter and condition of the fuel once you replace the primer pump. A real mess in the tank is possible.

Edit. I read back and see you got it to run for a few minutes. The primer pump is garbage from your description. Also it is quite normal for a diesel to smoke quite a bit after sitting around and loading up. That should clear soon. May as well throw that dirty air filter aside for the moment. You do not need it for testing and if the air filter is obstructed enough can make a diesel smoke as well. The engine just cannot get enough oxygen to burn the fuel cleanly. At least it ran for a few minutes so you have not got a totally worn out engine.