Welcome to FTE, like alot of guys will tell you here, does it got a stock airbox on it, ask to pull the intake tubes off and take a look inside to see if there was any dusting of the turbo, you'll be able to see chips or wear on the turbo fins, this is bad, shows it ingested alot of dirt. Check around inside the valley for oil or fuel, stright down in between the motor looking down in there, dry is good. Another point is to take a look at his service records if he wrote them down in the manual, not always trustworthy but people(like me) that take care of their vehicles write down all service procedures they do. Check all the fluids and look at and smell the tranny fluid, shouldn't smell burnt or look dark in color. Also take a look under the truck for leaks of any kind. Check belts and hoses. There's a few things I can think of right now. Good luck, seems like a good buy. Chris got you hooked up above.