Other than the general items (maintenance records, car fax, etc.), I would;
-check the blow-by, by opening the oil fill cap, while the engine is hot and running, and see how bad it is breathing in and out.
-check for any fluid leakage
-check how bad the rust is under the truck, especially the oil pan.
-try your best to see how often the fuel filter has been changed.
-Don't worry too much about oil leaking behind the engine. It's not the rear main seal, its the turbo pedestal (EBPV shaft).
-There are fault "soft codes" that do not turn the check engine light on. So make sure you try and get the truck pluged in to a good scanner to see if there is any faults to worry about.
I am sure there are other things, I just cannot think of them now.
At 200k miles, don't expect the transmission or injectors to be in the best shape. But pulling only 7k lbs, I wouldn't worry about the trans slipping.