Ok, lemme tell ya, my father got me a v6 auto stang when I was 16. I am 21 now, and I still have the thing. Its a 94 and currently has 132000miles on it. Yes, the 94s had gasket problems, but if you get one, odds are they've been replaced by now. I had mine replaced in 99 and hadnt had a problem with them since.
Ok, heres how reliable the car is. And keep in mind it is a 10 year old car, having me be its owner for 6 years...
First, battery problem. But they only last for 5 years if lucky
Second, head gaskets, but never again
Third, fluids, big big thing to teach your daughter about. I have had to refill oil and radiator coolant so many times, but oddly enough never transmission fluid.
Forth, once a year the O/D light blinks, clear the codes and it wont come back until next year.
Fifth, 7 flats. But if you live in a safe environment someone always seems to help out. I havent change a single tire yet! (know how to, just not strong enough)
Now then, most of these are maintence things. The only thing she'd probally hafta worry about is maintaning it, like the fluids I suggested. Oh oh, and get a convertible. All the guys will whistle at her, and all her female friends will want a ride. Also, chicks love accessories. I know, I am one. Get her a magazine showing her all the visual mods. Oh, but be careful, avoid the ones with bikini models drapped over the vehicle. They really piss off female mustang enthusiasts. We want to see the car, not her. Not to mention it makes us feel weird when reading it. Anyways, best of luck.