The confusing part is that the blue weapon (spectral rod) gives more DPS. But is this only the "weapon damage" and not the magic damage that you do? So going for the green weapon this increases my total intelligence so that would increase my spell damage?
Also where do I find my min and max magic dmg?
Items with +elemental damage or +%xdmg does this affect spells dmg?
Yes, INT increases spell damage.
Elemental damage increases it as well. I think + damage% is only for physical weapons.
As for which weapon to pick, it depends on your spec. Since you're Storm, you'll probably have passive talents that increase Cold and Lightning damage, which makes the blue weapon the best choice. Those passive talents also increase your weapon damage, if it is of the appropriate type.
For more info about the mechanics, refer to Titan Quest .