You won't be able to access any of the expansion content until you've completed the main story quests (which require you to reach level 50 in order to complete), so there's no benefit to buying it in advance since you can't access any of the newer jobs and/or zones without progressing through the main story.

If the next expansion releases (Shadowbringers - Summer 2019) before you're done with the base game, then you might actually save some money, since buying the latest expansion also seems to come with all the previous expansions as well. With that said, it probably won't take you that long unless you're playing VERY casually.