Expansions are all lv80 content, but they unlock daily login rewards which are helpful to accumulate useful things while you level.

If you buy the bundle, you get access to the first two (playable when you hit 80) immediately, and the new one when it comes out in 28th. If you only pre-order the new one, you'll likely still immediately unlock the account perks it comes with.

Whenever you're eligible to continue the main story, a green tracker portion appears on the top right corner.

Everything below lv80 is more or less a lengthy tutorial. I'd use that time to open the world (waypoints).

Once you buy the expansions, you can get a free 10% EXP boost from a guild. Some foods give you 10-15% extra, and utility consumables another 10%. There's no need to buy equipment until you hit lv80, you'll get good enough stuff from quests and drops.

Elementalist is probably the #1 class to make new players quit.