My friend has a 99 f150 with a 6cyl and the thing is falling apart due to rust. However, he's a lunatic and in 175k miles he's changed the oil maybe 5-6 times. He just adds oil when needed. True story.
The 4.2 was prone to some problems in the valvetrain as well as intake gasket issues. My brother in law had a 99 that he wound up having major issues with between 40-50k so he got rid of it. I had an 03 at work with the 4.2 that had some minor motor issues but the trans went at 60k. I also had an 01 with the 5.4 that was alright mechanically but was rusting apart, must have come from up north somewhere.
Had a 99 5.4l spark plugs blew out of the head and almost came through the hood with the coil pack attached. Brake lines rotted off the truck. Rust in all the structure of the bed. Trans began slipping.
Pinion bearings in the differential are problematic. Whining noise that changes depending on load.....turn steering wheel one way and the should changes. Coil packs on the sparkplugs go bad. Rotors warp and calipers STB.
If you get it really cheap and can do the work, might be worth it.