I think, the buy/sell in Armory should take minimal time, like...
5 seconds for armors
3 seconds for weapons and accessory
1 second for grenades
That means, you can do what you want, quickly, but you will be free to move after the minimal time since you used the armory (and depending what you did). With short key or not.
And, may be a global resources, like there is only 10 grenades available for all the team. Should depending about map.
We already have some limitation about accessory and weapons depending maps. Why not about how many available.
Stamina for Aliens sound great!