My first thought was to use steel studs on top of a pressure treated sole plate for total termite resistance and put fiberglass between them. Not a good idea given that seel would provide a bypass around the insulation. Also, I was surprised to find that fiberglass seems somewhat more expensive than foam. Finally, it may not be cost effective to insulate more than a foot or so below the ground level, which would be the top of the old foundation. A future project will be insulating the 1/4th of the basement that will remain a crawl space
While it would be easiest to install the foam on the inside of the studs, installing it on the outside would create two separate air cavities and thus a higher R value. It would also eliminate the problem of electric sockets piercing the insulation. If the foam has a foil side, does it matter which way it faces? If someone makes foam with foil on both sides, would it make sense to use it as there will be air spaces on both sides of the insulation? Is there a better way of doing this?