Diesel Bombers

just wanted to say hi and seek some advice. I live in oklahoma and i am a general contractor. I don't have my diesel anymore but i had a 2005 Dodge Dually 3500 SLT quad cab, with cold air intake, open exhaust out of the turbo, and a dr performance power module. i am looking to get another diesel but am a little leery of what to look for on a used diesel. i want a 2004-2007 5.9 liter three quarter ton with auto trans. That is based on what little knowledge i have which may or may not be entirely accurate. besides the aforementioned dually i had a '99 ford f-350 powerstroke dually. (hated it) injectors gave me hell. I love the cummins. my questions are: will i really get 3 or 4 mpg better from a 3/4 ton than a 1 ton dually, what kind of mileage is everyone else getting out of these 2 trucks? what should i look for when buying a used diesel to ensure that someone doesn't break it off in me by sending me home with a ton of problems. any other advice to help with situation would be greatly appreciated.
thank you in advance.
jay

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ps. i was also wondering if anyone could tell me what to expect with a dodge diesel that has around 100k - 150k miles. and does anyone have any info on how a 6" suspension lift will effect a 3/4 or 1 ton diesel in terms of wear on the truck and fuel mileage.