You can purchase the armor after completing The quest "proving honor" and being inducted into the companions. You purchase it from Eorlund Greymane.
The headsman armor has no codes, anything.
No - Light armor is leather, cloth, etc. Plate armor is considered heavy armor.
There is no Dragon Priest Armor, only their masks.
Armor points refer to how effective the armor is against preventing damage, the higher the number, the better.
you have to make it yourself
There is a mod for skyrim which allows you to give your horse armour (it makes your horse practically invincible btw), but without the mod installed, horses can't wear armour.
No, the actual look of your armor has no affect on stealth. However, the weight of it does.(the lighter the better)
There are Falmer helms, torso armour, shields, and weapons.
there is a way if you boost all of your skills over 100 and then disenchant it
It is daedric armour!
Yes, you can obtain the Nightingale Armor again in Skyrim if you lose it. You can do this by returning to the quest "Trinity Restored," where you can speak to Karliah in the Twilight Sepulcher. She will offer you the chance to reclaim the armor if you've lost it, allowing you to reacquire it without having to replay the entire quest.
Random Items, Daedric artifacts, Daedric sword with full blacksmith+ armor with blacksmith on it, then enchanted with armor with + enchanting