I just got my Nightingale Armour and I have like ~63 Light Armour, 55 Enchantment so I think I got the best version you can get from the start (without smithing upgrades) I also have 92 smithing with every perk but Dragon unlocked.

However when I go to upgrade the Nightingale armour and sword when I click on it, it only says Exquisite no Flawless, Epic or Legendary. Does this mean Nightingale armour is capped at Exquisite.. I think it's more likely I am just missing something which is why I am posting here, can anyone tell me what I need to do to unlock the Legendary upgrade for the Nightingale armour and Sword?

You need higher smithing as well as enchanting.

Most likely (not entirely sure as I haven't really played around with the Nightingale armor) you need 4 pieces of +25% smithing gear and 100 smithing level.

Ah, I had a mental blank on bonus smithing gear.. Hmm looks like I need to find some.. and level up my Enchanting, looks like I'm a lot further away than I had anticipated lol. Oh well more game time! Thanks for your help.

you need to get to 196 smithing to get legendary. use enchanted gear and potions.