If you can get an Oasis report from the dealer, it will let you know if the truck is a problem child. Knowing the coolant temp vs the oil temp will give you an idea if oil/EGR coolers are in it's future. You'll need a ScanGauge or somthing similar too see the numbers. White residue around the coolant cap is a good sign the coolant is venting from the cap and may mean EGR cooler leaking or head gaskets. Check front driver side of engine for fuel leak from fuel regulator. Most of these trucks have a drop or two of oil on the bottom of the bell housing but look around the engine block for oil leaks from the bed plate. Coolant condition is important. Ford gold coolant is.....gold and most ELC coolants are reddish. Either is ok but if it has green coolant it indicates truck was serviced by someone who didn't know enough to be doing it. Aftermarket intakes are another rookie issue. The factory unit is hugh and flow well, many aftermarket create more problems than power. Try to arrange to start the truck cold. Injector issues show up better on a cold start. Fuel injector control modules (FICM) are a common issue for rough cold start and voltage output can be verified with a ScanGauge or similar. Injectors themselves can be an issue for cold start and they are going for $250 or so EACH, if the rest of the truck looks good it may be a bargaining chip. Look at some pictures of the Ford oil filter cap. Many aftermarket oil filters don't fit correctly and can cause problems, an aftermarket oil filter cap would be a red flag for me (along with the green coolant). I probably missed somthing, but that's all I can think of, hope it helps. Sound like you got a good idea of what to look for on the rest of the truck. Happy shopping!