Seems to be a reasonable price for a good truck, mileage isn't too high, There was a problem with the early trucks and a poorly sealing airbox. This resulted in dirt getting into the engine and will show up on a compression test or by looking at the compressor wheel in the turbo
My truck also has 4.10's and mileage depends on how you drive the truck and how fast you want to go. The 4.10's are great when I hook to a heavy trailer but running empty leave me looking for another gear. When running in the 55-65 neighborhood I can pull 19-20, I actually got 18.5 comming out of SLC headed to Denver I took US 40 over the Rockies. Under normal driving conditions around 70-75 I pull down 15-16mpg. I also get around 15-16 when driving in the city unloaded.