Sandbox, definitely. Re which version, the base game is simpler and so easier to learn, but some folks argue that there's no point in playing anything but the latest version because it is 'complete' or 'more fun.' I think you'll get the most playing time for your dollar if you at least try the older versions first.
Whatever version you end up playing, you'll probably find old posts around here that address almost any question you might have. The local search isn't good at finding old results, but if you add site:forums.galciv2.com to a direct search from a site like Google, you'll get plenty of hits with no clutter from the rest of the net.
