Chuck,
I am a big fan of the Chevy/ GMC Duramax trucks. I have a 2005 crew cab / short bed. All the different Duramax engines have their own little quirks / issues. I would recommend finding a truck in your price range and then visiting one of the Diesel Forums. I hang out at
http://www.dieselplace.com they have a tremendous amount of information on all the Duramax engines with the known strengths and issues. There is even a section where someone will pull the build sheet and service record for your truck.
I can't comment on the mileage as my truck is not "stock" anymore but before the performance upgrades I was getting 20+ mpg.
Let me know if you need any more information.
Nils