Being a completionist, this game is a nightmare!
Maybe I'm crazy OCD, but you can make so much money doing tedious things. For instance, a shotgun sells for (i think) 360 and a pistol for 180. However, if you pick up a gun you already have it just adds ammo. This is inefficient, so I have to pick up one gun, sell it, go back pick up another, sell it, go back; etc.
It's boring, it takes forever, but it makes good cash.
As I said in another thread, the bias towards stealth is frustrating. Ghost, Smooth Operator and Silent Knockouts give more experience than any other method, thus making the whole "choose how you approach it" thing not really a choice at all. Given that, why do we even need weapon upgrades if the game doesn't want us to use them?