$25k, 100k, Dodge or GM
It would be a 5.9L Cummins, or a LLY or LBZ Duramax. Whatever nice truck you find.
Don't be scared of the 48RE. Add a TransGo shift kit and drive. If you ended up with a 6.7L/68RFE combo, expect less fuel economy, but better towing (power and extra gears). The 68RFE is a very good transmission really, and the 48RE can be built to be super tough, it is just a sloppy unit when stock.
Each different year CR Cummins between '03-07 had their little differences and quirks. The '06+ have a different interior than the '03-05.