You don't need anything except the core game.
You can have everything. You still need to pick and choose a subset either way when you build the agenda deck and side mission deck. For the first campaign you probably don't know exactly what you should be looking for.
You also could just play the Twin Shadows minicampaign as an intro. You would then have a little more idea about what to expect, which is useful when creating the agenda deck. (Some agenda sets are more useful than others with some class decks and some are must-include - High-Value Target is the best agenda card there is.)
Great advice! I'll definitely keep these options in mind. I should add that I fully expect to get additonal packs as xmas presents, probably after I start the campaign. If my understanding is correct, I wont be able to add any of these once a campaign is started so I'll have to run another campaign at some point anyway.
Cause well, I just have to right? And everyone will love this so much we'll just be burning through campaign after campaign right?