I would take it back home and get more advice and a few test drive. The only thing that I would worry would be the overheat for now, the rest can wait for a few test drive. Could be a dirty radiator, could be a false sensor, could be air in the system. Do you have a gauge? If not, by a scangauge for Diesel, plug it in and check the temp, if it is under say 200 most of the time, not going over 210 you are fine. If it is overheating, clean your radiator with a garden hose, just spray through it and check again.
I would not beleive the DPF is bad at that low mileage, I think they are trying to rip you off. These thing cost a fortune and they probably make double on it. Unless you have lack of power I would worry.
For the injector, I would try some seefoam. Go on a few Dodge Cummins forum and ask questions, read about black smoke, it could be just dirty and will eventualy dissaper, wait until you are sure there nothing else possible.
What's the turbo has to do with the problem you mention? What's wrong with the turbo? No power? No boost? Don't take their word. Get the scangauge and read your boost at full pedal to the floor, Does it spool up good, does it get to close 30 Boost, if so, don't touch it.
Did you go to a Cummins Dealer? And to a second cummins dealer to get a second advice? This is the only place I would go for Cummins problem, especially not to a MH dealer. Or like othe say, find a good cummins diesel mechanic, check cummins forums.
Good luck and get a few more advice before you throw any more money on this.