It's a Cummins 6.7 so I do not see where spending time in the shop versus a gasser is going to come into play. The 6.7 should be using DEF which the C&C Rams have been using for several years. This is a super reliable setup which is why Ford offers it to begin with. From what you mentioned prior you going to put 150,000 miles on the truck over ten years. As long as you wash the truck in winter frequently it should be in decent shape and have good resale value 10 years from now. If you can afford it you appear to be easily be in the cost effective range for buying an oil burner. I think it would benefit you to go on Cummins Forum and see what the C&C guys with 2008 and later 3500, 4500, and 5500 Rams have to say about the DEF 6.7 that has been in use for several years. Instead of speculating about it here where there are no DEF 6.7 users. I think you will find the feedback you get will make you feel very confident about the 6.7.