From what I've seen, Kawasakis are strongly-built, powerful, and comfy sleds. They handle pretty well, and they're dependable. You seldom hear of anything breaking on them at all. Re: the clutches, I don't know how good the clutches are on them; haven't heard anything (which might actually be good!)
The liquid cooled models- i.e. your Invader- are HEFTY and nose-heavy. This isn't a sled you can take off-trail at all, it'll shove its nose into the snow while the track spins. My brother-in-law had an Invader 440 for years and would ride all over New Hampshire on groomed trails, but he transported it to and from the trails by truck and avoided untraveled areas. He never had any trouble with his Kawi other than a couple unplanned off-trail "adventures".
If it needs a lot of work, $300. can seem like a bad deal real quick! But if the sled is in decent shape, $300. is short money.