What do i get out of buying the leveling guide?
You will get access to the full leveling guide all the way to level 60.
If you update the guide will I have to pay more?
No, all future updates to the leveling guide will be free from level 1-60 if you purchased the guide.
If i choose to do expeditions and get ahead in level should i skip parts of the guide?
If you get ahead of the guide, it is important that you still follow each step. There are a number of quests that the guide takes you through that are quest chains that lead to expedition quests, more side quests, and the main quest line. You CAN choose to skip side quests that you know do not lead to expedition quests, but you will have grinding steps later on, so ideally you continue with the guide to not have to grind as much XP during those grind steps.