Congratulations on your choice of trucks. Look for the usual things when shopping used vehicles. I always note things like tire wear, fluid quality (oil, trans fluid, antifreeze), and overall appearance. Look for obvious signs of body damage such as overspray in the fenderwells or on the undercarriage, and mis-aligned body panels.

The fact that the truck you're looking at appears to be stock is a definite plus. Look for evidence of prior modifications such as the contacts being scraped on the PCM, an uncaged lower pulley, or rounded corners on the four bolts securing the airbox to the intake tube.

One other thing to check....... take a good look at the rear u-joint, especially where it slips into the rear of the drive shaft. If it appears to have been beaten by a hammer, the previous owner drove the truck hard w/o any kind of traction control device.

Lastly, pull the VIN number off the dashboard plate (driver's side) and get a carfax report. You can also take the VIN number to just about any Ford dealer and they can pull a history on the truck to see if there were/are any warranty issues.

Good luck!