Love the game 5/5 it is awesome and love the idea the only thing i don't like is that there are way to many "buy" buttons. I can't blame you for trying to make money, but i think the only reason it is not as high rating is because of the "buy" buttons.
you should add a switch weapon feature because i had the boomstick and accidentally bought the sniper and was stuck with it trying to get the achievements which sucks a lot.
we should be able to upgrade a weapon multiple time. For example, the pistol could get more damage, dual wield, then maybe the bullet can travel through multiple zombies.
Lots of places to get extra stamina/energy through boosts and daily rewards.