As far as I'm aware weapons don't relate to levels. everything levels up as you use it so you can infer that every weapon/item of equipment you find should be level 1 and it will level up as you use it. That would mean that all classes of weapons/equipment would be available for you to earn or find. I have no doubt some classes will be for sale in the tower but they would likely be lower classes e.g. basic. The more exotic ones you have to earn or be really lucky and find.