frame. thats the biggest one. if the frame is bent at the rear bump stop. check the tailgate gap to see if thats even.
+1 to frame
mostly everything else you could replace.
check for:
-run the A/c max for 5 min (you do not want to deal with heater cores)
-run the heat for 5 min
-with truck running and not moving turn steering lock to lock, listen for noise
-during a test drive go about 20 mph and rock the steering wheel back and forth to check for worn ball joint (noise)
-jump up and down on the rear bumper listen for squeeky bushings
-if you can get under the truck and see if there is any oil resedue caked near the drain plug and bottom of tranny
-open the airbox and check the air filter, if at a ford dealer or like you can somtimes have them "throw in" a new filter if the one in there is rough
-check coolant level, if its low might be a leak,
-check windshield washer fluid tank if empty it might leak
-turn all lights on exterior and interior look for out bulbs
-look for at the hitch, if it looks like it was often used vs not at all, can give you an idea of if the truck has been used to tow.
-check the seat plushness and leather condition
-flip all switches knobs and turn on/off all settings
-make sure 4wd works in every mode and you dont get a check 4x4 light
drive 10 ft in 4wd on pavement youll know if the front wheels are locked
all these things may seem petty but if you do each and they all check out then at least you know someone might have taken good care of it. also these are little things youll kick yourself on if you get it home and the wipers need to be replaced cause they dont have any rubber. with the price range you are looking at you are going to be looking at a vehcile that is near 8-10 years old. raptor and mile aside there are small things youll want to look for.
ill say you will likely be better of just not worrying about if the shocls were rebuilt. just look at the overall external condition and just tell yourself now to spend the $1k to get them done. you never know. somtimes people rebuild shock cause they have a rough ride but it turns out its a dent driveshaft or somthing else and they are tired of chasing the issue so deside to sell it....
overall id say look for as stock as possible in the best overall condition (all the little things) it will tell you how the truck was taken care of over the years.