Even if you don't invest in stamina, 300 capacity is more than enough? You need to do a quick value:weight calculation in your head, to see if the item is worth taking. I usually go for anything with a ~10:1 ratio, at higher levels I will probably go with ~30:1, as money is easy to come by.
The lord stone is the one I used, magic resist and armor rating is more useful IMO.
They do if they sell for over 1000 septims a set.
With 100 speech you sell items at half their value, unless you have speech perks (which I don't recommend). So what armor is this you are finding that is worth 2000? Ebony armor is so rare to come across even at my level (60+).
Its a waste of space carrying stuff like that about, considering you can make potions worth stupidly high amounts. Giants toe and wheat is the best combo I have found, throw in another ingredient that compliments one of them, and the potion is worth 4000.