more than miles, ask about maintenance records. also ask about oil change intervals. oil changes are critical in a 7.3.
items to check:
look at the turbo compressor wheel, look for chips and feel for shaft play
look for leaks
check the color and smell of the tranny fluid
try for a cold start (when the engine has been off all night) see how well she fires up
jiggle the pulleys, espicially the water pump for pulley play
test drive, warm up and get on it, hows it feel?
be sure 4x4 works
place the oil fill cap upside down on the oil fill hole, while the motor is cool and running, if the cap flips off there may be too much blow by, if it just vibrates off its fine. poor mans compression check.
i would say anything under 200k or 250k on a well maintained PSD is lower mileage. thats JMHO