The 84 and 85 models are mechanically identical, so I'd be amazed if anyone ever raised an eyebrow over an 85 model being used in VMX or Vinduro.
As Leith said, its hard without pics, but $1500 sounds fair. Original sometimes means "flogged to hell and back, and never had a spanner laid on it"....
Unless its immaculate, it still isn't a bargain at $1500 - these bikes aren't valued all that highly at the moment for a variety of reasons: They're well behind the class leaders as a 250 VMX bike, and the whole Vinduro thing is still quite a new concept. Its weird, because the ITs have a bit of a cult following (and consequently higher prices) but the KDX the better bike overall.
The main thing against owning a Kawasaki is the spare parts situation. They're like a lot of defunct Euro brands in that you can get the parts, but you can't rock up at the lcoal dealer at 4pm on a Friday and expect them to have it in stock. You need to scour Epay, make contacts, be willing to buy from overseas etc. I am also yet to find a Kawasaki dealer with a parts department worth pissing on, although Cowra Motorcycles are looking like changing my mind as they're willing to search for the bits I need (and they didn't tell me that there was no such thing as a 1984 KDX like the Canberra dealer did....).
Having said that, they're great bikes - to get a better bike for Vinduros, you'll have to be a bloody good rider or spend a lot more money (or both).