$600 isn't a bad deal, but it's not a great deal either.
I'd keep a lookout for one that has a pipe or carb / other aftermarket parts. The hulls are cheap, and if you want an X2 that's any fun to ride, your looking at adding at least a Pipe ($250), Probably a bigger Carb($150), Head($100), etc... The aftermarket parts add up really quick compared to the Hull price.
Around Michigan, I'd pickup a bone stock ski for ~$400ish, or a modded one for ~$800, but I'm cheap... heh
A friend of mine picked up a 92 X2, WC Pipe, WC Head, Waterbox, SBN44, on a trailer for $600. He had to drive 800 miles round trip for it, but it was worth it.
-Rob