You'll need one starter deck per person, so if you're looking to play with 4, that's 4 decks. It may pay to pick up both combo packs as this will give you & your crew a bit of choice and room to experiment.
I picked my deck purely on the basis that my city, Adelaide, is one of the cities in the game. Others are picking decks based on matching their interests. It's up to you.
The combo packs also come with bonus promos, which if you're a junkie, you'll surely want.
The expansion B decks are out, but get a few plays of the starters under your belt before worrying about these.