It's potentially a giant headache and the price needs to reflect the risk premium. As a point of reference, last week I test drove a 2007 supercharged full fat Range Rover with 91,000 miles, complete air suspension replacement, new tires, fully documented dealer service history, absofuckinglutely immaculate. Asking price was $15,000. Granted, that is an older model with the 4.2 instead of 5.0, so not a perfect comparison, but something to consider. Keep looking, you'll either find a better deal or an example in better shape.
Another way to look at it is "How much am I prepared to spend on repairs if I bought it?" If your budget for bringing the truck up to your standard is $5,000, then you could alternatively shop for something in the $16,000 range that doesn't have overheating issues. Maybe you'd get lucky and repairs would be cheap and you'd end up with a deal. But it's a gamble and the outcome could also go the other way. Depends on your risk tolerance.