What year? The 02 and up are what is called "Job 2" and fixed quite a bit of the first model year 01 problems. 4WD or 2WD? XLS, XLT, XLT Premium? Rusty?
These trucks can run nearly forever with the proper care. Things to look out for are timing chain noise (probably the biggest thing to watch out for), a shift flare between 2nd and 3rd gear (indicates trans probably wasn't taken care of, but usually not too expensive if caught early) emissions codes (bring a code scanner when you go to buy it to check if there are any stored, pending, or drive cycle not reset codes), inspect CV front axle boots and CV driveshaft boot for tears or flung grease, and ball joints/tie rod ends/abnormal tire wear. Lots of people don't ever touch their suspension, so any abnormal tire wear could indicate ball joint and tie rod replacements. It's not too expensive to do though, even using top of the line Moog parts. Other than that, not too much.
These trucks are very reliable, and have been proven since 1995. The Sport Tracs are basically an extended frame second gen (95-01) with a bed instead of cargo area. The frame is also reinforced and stronger than the standard second gen as well as having a longer wheelbase. The Ford 8.8 rear axle is tough as nails, and even the Dana 35 SLA front axle is pretty stout. The engine can go for a long time with proper care (synthetic oil helps out a lot, and I really like the Valvoline MaxLife stuff), the trans is pretty tough with proper fluid changes, and the transfer case is the proven BW1354. All in all, very good little trucks.
