I'm a bit confused about how this LCG works with respect to expansions. I get that a campaign is made up of a deluxe expansion plus several card packs. Is the idea that you buy the deluxe expansion and the several card packs before starting the campaign, so that you can use all those cards in deck building? Or is the idea that you buy just the deluxe expansion, play through its scenarios, and then buy a card pack and play through that scenario, etc.? I don't really get how the second way would work though: none of those cards would be available during the initial deckbuilding, and so you have to buy them with XP. It seems like you'd only get try a very small number of them that way.
I've played only a single game of the core set, so I'm pretty novice at this.