Pay attention to how it shifts - that's the biggest piece of advice I have for anyone considering a Ford with an auto. Delayed upshifts, held gears, general sluggishness can all be signs that are easy to spot. Sometimes problems manifest themselves right away, sometimes it can take a good warmup to notice it. I drove a Cougar off the lot once and three hours later the tranny wouldn't stop slipping, but it didn't show up on a test drive. Listen/feel for bearing noise or roughness, (can feel fronts when braking and turning) have someone sit in the third row (or middle) and listen for noise from the differential.
Something I just found out on my own truck - look for unusual stains in the carpeted trunk area, especially if there is a "different" smell in the truck. I thought it was "old man smell" and that the stain was just dirt/soil. Boy was I wrong! I shampooed the interior and no matter how many times I did it, it kept smelling like a landfill! Turns out the previous owner had a cottage, maybe without garbage pickup. They apparently took their bags home with them and some leaked. When I bought the truck it was winter, and the small individual stains had long since dried. Now after shampooing, I have removed my third row and am trying to find a way to clean it without LAUNDERING the entire covering (not good for the leather/vinyl!)
Also, check for idle quality and if you can see the individual ignition coils on the plugs with the 4.6. You didn't post the miles on the truck, but these are a known fail component on these engines, which can fail at virtually any time. Dealer price on them is $70 each (x8!) but cheapo aftermarket pieces (which sometimes only last a month or so) can be had for about $30. Just something to keep an eye on. If you have a handheld code scanner, you can check if there are any stored codes when you see it - they might not have triggered the CEL but would be stored anyway.
Check the rocker panels and the dog legs (body and door, especially on a Limited or EB). There is also a thread on here about a ground wire that can chafe and ground out at the spillway for the oil filter (one of the sticky's at the top of the 2002-5 stock list)
Good luck! And have fun if you get it!
