The 3.8 are not garbage, The do pretty much require a head gasket change at about 70k Once that has been done they are good for a good while. The gas mileage is not super but they are 20+ so not horrid. The v8's of these years are pretty lame as well hp wise (not that a kid needs hp any way)and the parts cost a little more. A v6 stang is better than most peoples first car, Plus most all Gt or cobra stuff can be added to make it look the way he wants and that will give him a goal to work for plus you can help him do the little installs (wheels, ground effects, dual exhaust, etc)
The perfect example is Cam's mustang that they made a fobra out of. it started as a v6