Just bought the game and and very impressed so far.
If you haven't bought the Crusade Expansion, I strongly recommend that you do. Longer story short, GalCiv3 is a whole new game now, with that expansion.
On the other hand, Mercenaries and Intrigue, I'd pass on them for now if I were you -- there's plenty to learn and do without them, plus Intrigue doesn't have nearly all its "new-ish" kinks worked out yet. Others will disagree with me on this point, and you may as well. I just figure keep it "simple" for starters.