I had an F250 extended cab 4X4. Great truck, but it had some serious problems with the coolant system. Inspect around the water pump and timing chain cover for ANY sign of leaks. For some reason, the aluminum corrodes easily and they're a cast iron bitch to replace. Between the timing chain cover corroding and a stopped up water jacket 'somewhere' in the engine block, my truck would overheat in the winter...in Michigan. I bought it to tow my boat and never could until I did a complete tear down/overhaul of the engine. Make sure you test drive the SHIT out of it and if something isn't right, unless you are absolutely sure you can fix it cheap, WALK AWAY.
(oh, and it only got about 7 mpg...something to think about with $2/gal gas)
edited to appease the spelling nazis