Hi John:
Depends on your application I guess. Most kits have the sole being the same leather. If you are going with 6-8oz that should be strong and thick enough for the sole. If you feel you need it thicker, you can double up, stick it down then cut away the seam allowance.
Do you plan to wear these while hunting or hiking?
I just looked up this post on etsy where the craftsperson used 4-5 oz leather. You can see that they "sag"
http://www.etsy.com/listing/59130786/suede-boot-moccasin-inca-ankle-high-1000
I think you would have better luck with the 6-8oz. Elk and buffalo is rather stretchy so remember to fit these snug to they don't stretch out and get sloppy. You can also get deer tanned cow hide that is nice. I think I got 17 square feet from "The Leather Guy" on ebay for around $60 shipped in October. The deer tanned cow hide was 6oz and it's still stretchy but not as much as I understand the elk and buffalo are.
If you have the means you might try making a prototype pair from some canvas, old denim jeans, etc... something sturdy and firm, so you can get the feel of how the pattern fits before cutting your leather.