If you are looking at an older truck because of price, shop around and you might find a good deal on a newer truck like I did. I recently purchased a 2008 F-250 with the 6.4l diesel (the first year of the newer engine) with 109k miles (low for a diesel) for only $11,900. It was a program (leased) vehicle, so I know the required maintenance schedule was followed as it is paid for as part of the lease agreement. To give me some extra peace of mind, I purchased a 24 month 24k mile extended warranty for $1,000 - just in case, as you never know buying a used vehicle - even from a reputable dealer as I did. Though the 6.4l engines don't have the reliability out of the box as a 7.3, they have twice the power (350hp, 650 lbs/ft torque out the box) and considering all that power, the mileage is not that bad (I typically get low 20's on the highway and high teens 50/50 driving). It can hit 27 mpg cruising at 55 on level ground - until the regen kicks in. Plus you are much more likely to find a lower mileage newer truck. Remember, things other than the engine wear and age over time too, especially rubber parts in an older, truck if you are lucky enough to find one with relatively low miles in decent shape. There are tuners and kits available (for a couple grand) to improve the MPG, reliability and nearly double the power output (if that's your thing) of the 6.4 when combined with an EGR/DPF delete (warning deleting these emission items is technically illegal, but many states don't check.)
Chip