Returning, Season 2 question
It sounds like you might have a couple of the episodes already unlocked. If you logged in, even for a second, between July 2014 – January 2015, you would have unlocked whichever episode was currently live.
You can check to see if you have any unlocked by going into your Hero panel, and clicking on the “Story Journal” icon on the left side and clicking on “Living World Season 2”, and if any of the 8 episodes listed don’t have a padlock next to them, then you have those episodes unlocked.
So when hovering over the item in the store, it will only show you the episodes you have yet to unlock (and it should recalculate the price accordingly).
As for the second question; the Living World Season 2 was likely the most important thing to buy (especially if you love stories) as HoT will continue on the story and encounters of Season 2.
Otherwise, there isn’t anything that is a “must”, but the general things like bag slots, bank slots and other little upgrades that you might need (bag slots if you are going to create a Revenant etc.).