Do I need to buy FFXIV:ARR if I already own FFXIV?
If you played the original Fourteen, you already have an account, and all you need to play (even with your old character, if you like) is go to account management and subscribe.
Having Legacy status (obtained by being subscribed to the original Fourteen for three months) gives you a permanent discount on ARR subscription.
In either case, if you already had an account at ARR's release, you should have gotten two weeks of subscription for free. As of this writing, you should still have a week left: try logging into the game.