After doing a side-mission am I allowed to look at the next Story Mission (since it's already decided) before buying Agenda/XP cards?

This might be up to the group to decide. Personally, I wouldn't. The next mission may be decided, but it is not setup.

If you do, the Heroes should also have the same advantage.

Technically, the Imperial Player has access to the entire Campaign Book and can read it at any time, so you should already have some general idea of what is coming next. But since the Rebel players choose the next mission, and you need to make your purchases during your upgrade stage, it would be bad form for you to start digging though the campaign book to maximize your upgrade path.

Mission Stage: RRG, Page 19:

Choose Mission: Rebel players refer to the campaign log to determine their next mission. If this reads “Side Mission,” the heroes choose one of the active Side Mission cards. If this reads “Story Mission,” they must choose an active Story Mission card.