Indeed, Jim is right, XM1 and 2 both have the right mounts for attachable lenses but you cant put completely new ones on, not that you'd need to, the lense on them is superb.
XM2 is what I've got and it's a great camera. However I do still say that editing and riding will always make a DVD over how good the quality is.

Although having both is obviously a bonus.
Just think long and hard about whether you really want to splash out that much on a camera. Have you thought about editing software as well?