I see no concern here, 6.0L are ok after 2005, Highway miles are easier miles than city. Cold is no problem for the newer engines, like you said, but i would still plug it in if under 10*. 125,000 is just broke in for the diesel. Go to your local Ford dealer and get an OASIS report on the truck. take the VIN to them. It will tell you all the warranty work that has been done on the truck at least. If the owner kept taking the truck to ford after warranty ran out, all that information will be there as well.