I've had two different 2005 Dodge Ram 2500 diesels. I loved the first one so much that I bought a second with a quad cab due to an impending baby. With the 2005 you don't have any smog crap (except for a catalytic converter) and no DEF. With my first 2005 I had 140k miles when I sold it and it had the original brakes and had over 50% more to go. I change my oil every six months or every 7,000 miles, whichever comes first. I keep them stock for the most part, but I just ordered a 2 micron bypass fuel filter and a fuel tank vent kit so I can get 3-4 more gallons when I fill up (can't have too much fuel!). With both trucks I've never had a single big issue. The only common issue between the two is the mult-function switch (turn signals, brights, etc.) and the radio. For mileage the overhead will lie to you all day long. Mine says 21-22 mpg half the time, but hand calculated it gets 17-18 mpg running I-5 at 78 mph. If you can keep it at 65 it'll get 19-20 mpg. If mileage isn't a big deal to you the Hemi will be fine, but I image it will get really bad mileage with a FWC in the bed and towing a heavy trailer.
If you want a diesel try to find one of the earlier models with the 5.9 HO that only has the catalytic converter on it. I think these are 2003 to early 2006. I'd stay far away from the newer models with the 6.7 and the particulate filer. There's a reason why the cubic inches increased from 5.9 to 6.7 when they added the filter!
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