Does anyone know if there are Cat 517 models out there?
no but norscot use to make a cat wheeled 545 cable and grapple skidder, you can still get your hands on a cable 545 on 3000toys.com but the grapple skidders are pretty rare now,
JOAL made a 518 skidder http://www.constructionscalemodels.com/productdetail.aspx?recid= 2266 which is still available in 1/43 scale. It looks OK with other 1/50 equipment.
I was thinking about building my own 517 skidder but I'll check into the 545 and the 518!