I have a 99 Ranger with the 3.0 (mines a flex fuel so it can run on any blend of Ethanol/gas). I have 140,000 on it now and it has done me very well. Overall it's very reliable. Its only been to a mechanic twice in the 100,000 I've put on it. Once for a new clutch, and then recently the performance chip on it died and fouled it for a while. The biggest factor will be how the previous owner cared for it, and how your son cares for it.
I think motor is underpowered, but I have 31" BFG's and live at 9000 ft and have 150 - 300 lbs in the bed at all times. Overall I think the 3.0 is a fine motor. But I've always wished I had the 4.0. My mpg is about 15 usually.
I think Ford made a lot of 3.0s in various forms so I think they worked out the kinks pretty well by 99. Just speaking from my experience with it.
Is it 4x4, extended cab, any other add ons, and what price are they asking?