I have an '03 Z370 that I bought from one of my dad's buddies a few years ago to use as a 2nd sled for the GF or whoever. It's been good to me so far, with only about 700 miles on it total. (had 200 when I bought it). I don't believe there are too many things that can go wrong with the little fan, so I think the reliability will be good. (I had a MC570 that was good to me too.)
I will say that it is still a full size zr chassis... Kinda big for a really little guy, but my friend's girls rode it last winter with no problems, and they're probably not much heavier than 85lb. It'll hit all of 60mph too, so that's a big step up from a 120.
If you're looking to go cheaper, what you see kids riding a lot around here is old Yamaha Enticer 340's. They're a nice 3/4 size sled, and easier for a smaller kid to handle, and work with. From there, you can move up the a full size chassis 370 or 440.
I'll add that I have taken a ride or two on the 370, and though it's no powerhouse, it'll get you there and it's light enough to be flingable. I have a good time bombing it aroun. It sounds really angry too... Tiny muffler.
Oh, and it is a fuel sipper, which is nice. I'm converting it to hydraulic brakes, but the cable brakes aren't bad.
Oh, check out
http://premierrpc.com/Premier_home_page.php for new 3/4 scale snowmobiles geared toward kids/smaller framed ladies. Seems like a great idea to me!
Chris