Check out this item I found on eBay:
Limited Edition Staff only 54mm Chaos Knight model 2003 boxed w/ certificate
Link: http://www.ebay.co.uk/itm/Limited-Edition-Staff-only-54mm-Chaos-Knight-model-2003-boxed-w-certificate-/140641008835
Can't think there would be that many of these about. Anyone willing to put up that kind of money?
That's not a bad price. If the model was just a bit nicer then I may be tempted to pay it. From what I hear though the knight is a touch small in stature...
I forget what I paid for the limited edition female wardancer I got on eBay, but she wasn't cheap. I've had her for a few years now and have been planning to convert her into a Harlequin. I really ought to get on with that project, or else sell her...
Knowing it was a limited edition, I don't know if I could start hacking it up for a conversion. Gw must be a great place to work if you're in need of something unusual to convert etc.
A possible alternative might be the 54mm Valiant version of Frazetta's Death Dealer:
http://www.hobbybunker.com/products/mounted-death-dealer-4875
http://www.ebay.com/itm/Valiant-Miniature-Kit-9862-Mounted-Death-Dealer-/360393693075?pt=LH_DefaultDomain_0&hash=item53e9235793
I don't have it & can't seem to find any pictures of the actual miniature, but considering it's not too expensive it might be worth checking out. I've been meaning to for ages, but now first need to build other stuff.
Valiant miniatures are an older 54mm line, but still ok. Certainly if you're looking to cannibalize them for parts or want a body to base a conversion on.