Nothing wrong with the 2007, especially since it has the "tried & true" 5.9 Cummins engine versus the 6.7. If you are interested in this truck, get a VIN history on it to tracks its history. As for mpg, I got 15/18 (city/hwy) mpg on my truck when it was stock. Now with the Smarty and more miles on it, I am not getting 18/21 (city/hwy), when running 75 mpg on the highway. I agree though, if you are changing for the mpg, you won't get youir money back.
As for trade-in, I checked NADA resell prices, and the value for my truck was slightly more than a equivalently packages Duramax or Powerstroke.

Good luck with your decision...just remember, you can get a lemon with any of the top three, just do your homework, by investigating a head of time!