Before I bought any truck, I would dump the ECM and go over the 30+ pages of data (a complete dump, not a summary), I would have the truck put on a dyno and checked, I would pull oil sample from engine and have it tested, then I would then go over the truck bumper to bumper and underneath. Paying a good mechanic (not affiliated with the dealer or owner) to go over it with you would be money well spent.
With the way trucks are selling, taking the time to do this most likely will not risk having the truck bought by someone else before you get all the results. Try and negotiate the dyno testing and other costs of inspection be paid by dealer. That will be difficult, but can be done.
If dealer doesn't want to jump thru these hoops, then there are a lot of trucks out there. Move on to another.