At the current time, you will need to purchase both the "original" (2.0) FFXIV and Heavensward in order to access all content.

That said, you only need FFXIV: A Realm Reborn to level to 50, and can purchase Heavensward as you near level 50 so that you are able to gain experience towards level 60 as you go through the large number of ARR story quests between the end of the 2.0 story and the start of the 3.0 story.