it could be the water pump, i've seen impeller blades get wore down due to rust in the cooling system. i've also seen impellers become loose on their shafts. these problems will not cause the w/p to leak. however, as Turtle mentioned there are many other reasons for overheating. some can be easily/inexpensively fixed, some can get very expensive. the 5.0 isn't known for blowing head gaskets, but if the cooling system has been neglected for 13 years (as many have been) and if the engine has been severely overheated, you could be looking at expensive repairs. i sure wouldn't take the word of the person selling the vehicle as to what the real problem is. if he's a dealer, make him change the water pump, thermostat and flush the cooling system. i'd also pull the plastic cover from the bottom of the radiator and check for signs of leaks. if after that it doesn't overheat, consider buying it, but there will no doubt need to be more maintenance/PM things needed. if you also have to replace the radiator, rad cap, hoses and serpentine belt, that's no big deal.
as far as radman's comments... all true, and i'll add one that took weeks to figure out on a '54 Corvette. the problem turned out to be a baffle, in the bottom of a resently re-cored radiator, had come loose and was blocking the coolant flow. the engine could idle all day w/out overheating, but if you brought the RPM up a little it would immediately overheat. let it idle again and it was fine.