I don't know the best way to tan leather, but I figured out quality is worth the high price. I think all of the brands you listed are good ones. As long as you are talking quality leather all the top brands are comprable. It comes down to personal preference. I started with some cheaper stuff and am now using leather from GJL. He from PA and goes to a lot of our big matches and I liked his work. Ask folks at local matches and look at some of the rigs they are using. Most cowboys love to talk about gear almost as much as guns. Don't overlook used stuff. Sometimes people will switch styles of holsters or go to some fancier rig, and they will get rid of the old ones.
You should be fine with the grips. The main concern of SASS is something like the Houge rubber grips or a big target style. I can't imagine somebody would give you a hard time over any grip that came on an actual Colt. I think the very early ones used a hard rubber which looks pretty close to plastic anyway.