Same, my XJ was, and still is my first car. I bought it back when I was 16, and its still going strong now that I'm 19. Well, running that is, not driving at the moment, but that was an offroad mishapp, and hopefully will be fixed before the weekend.
I got a '99 cause I saw it on a lot and fell in love with it. I would vote for a '99 because I love the new body style, you can find some in good condition for relatively low money, HP D30, coiless ignition, etc.
Before I put my lift on, I got great gas mileage. I was driving 15 miles one way to school every day, to work on the weekends with the same distance, and then some every day after school and at night on the weekends. Before I lifted mine I was able to go two full weeks on one tank.
I also take some offence to the comment about getting the 231. Not all kid drivers are retahded like most adults think. I've worked on my car so much over the period I've gotten it, from tune ups, oil changes, fluid changes, to full brake jobs, helping on my initial lift, then putting in new leaf springs by myself, and then removing my front axle then replacing it by myself, SYE and driveshaft. My mom once told my dad to come help me. He replied by saying he couldn't, he had never even done any of the stuff I have done to my Jeep, so he wouldn't have been any help. He has been impressed with most of the stuff I have done.
I couldn't wait to get my licence because it signified freedom for me, and I could go wherever I wanted, when I wanted, and I didn't have to wait for my parents to drop me off. For this reason I learned everything I could about driving. Also, its not like kids don't have common sence. (don't make an arguement about kids going off and drinking and driving or doing drugs, because you as well as I know that adults do it just as much. I know some kids who have even smoked with their parents before) If he gets a 231, then simply tell him not to use it on dry pavement, and only in snow or offroad like that, then if hes not a complete knucklehead, he will listen. He will probably have a sence of pride in his brand new to him car that he won't want to screw it up that fast. I'm sure he will probably know how to use 4 wheel drive better than some soccer mom or lady like that in her Mercedes SUV that thinks because of all the money she paid on it, and that it has 4 wheel drive, nothing will stop her from getting to the store, and she can brake just as easily in snow and ice than she could if it were sunny out.
As for the power, that thing does have some power. Here and there I have had a little fun, and I have been able to burn my friend's Saab 900 off the line, another friends Audi, an old school oldsmobile, and even a friends Porsche 944. (Yes, I infact have burnt him off the line with my lift and 32's. He did come back in a couple of seconds to compltely smoke me, but I did take the lead for a while).
I've had many good times in my Jeep and its always been there and never let me down. I would definitely recommend it.
But back to the XJ. I would definitely do it, and you'll have some security to know that if it does start snowing out or its bad out, he is driving a safe reliable and proven SUV that you yourself have knowledge and experience driving.