Sorry, I have noob questions and am very excite.
How much loot can you carry? Can this amount be upgraded?
Not sure, but I've never been 'encumbered' or reached the inventory limit in the beta.
Clothing and armor, are they separate? Can I stick with the look of one design , but keep getting different gear?
Yes, they are separate. Armors still have their own visuals though, but kinda barely noticeable.
Weapon skins, are they exclusive to a weapon type?
No, you can apply them to any weapon type.
How are grenades, ammo and I assume health packs handled? Do enemies drop them upon death, or do you buy them from a vendor?
Enemies drop them sometimes, and you can also replenish them in weapon caches.
So weapons deal damage in an RPG sense, are headshots critical damage zones? Does shooting a dude in the foot do the same damage as shooting someone in the chest?
Not sure, I turn damage numbers off all the time.
Are weapons customizable, or just randomly generated?
Weapons are moddable, if that's what you mean.