I'm kind of surprised you can't get a whole layering system from Sitka, but there is a lot of good advice already listed. I am a fan of the Sitka line and have used the brand for years in some nasty terrain, but they are kind of expensive.
I would recommend some good glass (as others have) and also a good rangefinder with angle compensation. The western hunts may present some longer shots than you are used to.
As for water, I use a Katadyn My Bottle purifier bottle and it works OK. My beef with it is the straw/hinge isn't designed well IMO.
Hope that helps.