First suspicion would be that the fuel injectors need either reconditioning or replacing. Rough running, hard starting and excess black smoke are all classic symptoms of worn injectors. That said, the modern turbo-diesel is a pretty complex animal, and you could also have a problem with the emissions system (EGR valve) or maybe a build-up of soot and oil in the intake system (which could also affect any number of sensors around the engine).

Don’t forget the basics, either; something as simple as a dirty air-filter element can also cause problems just like the ones you’re having.

If you are not sure get another opinion. That would probably mean having it trucked to another location, but that’s the best thing to do to make sure the advice you are getting is correct.

I would get a second opinion on the Kia's engine before you do anything else. A sludged-up engine suggests a lack of servicing, but if no damage has occurred you might be able to salvage it. I wouldn't recommend either a Multivan or Valente, those you are looking at are too old and potentially too expensive to run and repair. A seven-seat SUV might be a better option.