The shack is next to the Northern fire (Communications Tower) in Soviet Installation. You can't miss the symbol on your map.
To buy all the items, you need a lot of coins. You need to collect pretty much all of the coin caches in the whole game plus complete the mission for completing all tombs. That mission will get you a lot. You also need to look out for loose coins. I only had one cache uncollected after. Each cache is 25 coins. I had 42 coins left.
In each area, Monoliths show the location of coins in the ground or just look out for a silver shimmer.