I can't see them any more because I've purchased them, but I see Godslayer-related stuff under "Bundles" and Turan stuff under "Account & Character"
Two. Neither are required to reach max level.
- Savage Coast of Turan (which offers another zone for level 50-55 and another zone for well-geared level 80s, plus some dungeons and raids)
- Rise of the Godslayer (which has a zone for level 20-40 and then many zones and dungeons for 80: i.e., the vast majority of the level 80 content in the game, basically).
Level 20-40 for the first Khitai zone, and level 50-55 for the first Turan zone. However, again, neither of those are "needed to progress." The only "must-have" expansion stuff is Godslayer at level 80 because there's nothing else meaningful to do at 80 but a couple old-world raids, and because of the AA system which is the main way to progress once you hit the cap. The Turan stuff is also nice at level 80 because it has a couple of the most fun and rewarding solo dungeons in the game.
- Turan: You will still own and be able to enter and use normally even if you go back to free-to-play.
- Godslayer: You will still own it but you will not be able to use until you go premium again. When you do go premium again, it will be right where you left it.