I've made hi top moccasins before. Are you going for centre seamed eastern style mocs, or more of a 'plains' type, with the soles stitched on in what we now think of as a typical moccasin style?
In either case, don't go too thick with the soles if you want them to be authentic. They should be soft soled so you can feel the ground beneath you, but thick enough to protect you from pointy things.
You can use any leather, and I wouldn't go too expensive for your first pair until you see what you like/don't like and then you can make the second pair put of better stuff. If you want breathability, brain tan uppers are the way to go, but that's expensive. Veg tan will do just fine for the soles.
Of course, if you want to go for a more modern style, then any leather supplier will be able to sell you thick leather for soles, some even tan stiffer leather for this purpose.
If you don't already have a pattern I would suggest buying one to help you make the first pair. It's worth the ten quid to give you a good understanding of how they are made.
