never serviced it, I would say that's your problem right there, diesels are not like petrols that will just run badly if you don't service them, if you don't service a van it will stop and never run again.
you need that oil changed at least asap keep good oil in the van it lubricates the engine and is the blood of the car, you will notice a lot of difference after a service, the soot is probably old oil burning poorly, cleaner newer oil should sort that problem, the fuel filter will help as well with cleaner running, and the nice new oil that you put in will flow smoothly through your new oil filter,
you are probably looking at around 200 for a decent garage to do a good service on it, you really need to do this