My vote is also 6.4 (of the 3). Most refined interior, most power on tap, reliable (if you do all your maintenance, and do the basic DPF delete, EGR delete and off road tuner).
7.3s are great, but because no one can get past the 'almighty 7.3' mindset, in my mind the price is obnoxiously over inflated.
A 6.0 is gonna net you better fuel mileage, but again to have reliable you will have to mod it (and the mods will be more expensive, ie EGR delete, head studs, etc... I don't know exactly what goes into that as I haven't actually bulletproofed a 6.0, but there is plenty of info in the 6.0 sections).
As far as 6.4 price, it depends on what options you want and how patient you are. There are some bare bones trucks in the teens, and I got my King Ranch for $28k with 58k miles on it (but I wouldn't take that as usual, I think I pretty much stole mine). With some realistic expectations of what you want and a lot of patients you can find something in your budget, just keep in mind you will probably spend on the $2k side in the basic bulletproof after you buy it.
Also on a last note, I don't know where you live, but if your state has diesel emissions testing, I completely change my vote to bulletproofing a 6.0!
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