I'd definitely recommend the community balance patch. It completely overhauls a lot of the game's mechanics though, so it's a decent idea to play with just the official expansions for a bit to see how it changes the game.
Ideally wait until one of the aggressive Steam sales and get the Complete Edition for ~$10 (even $30-$50 for everything would be great given the amount of content).
Several others that add more civilizations are good as well. However, again, the cost right now for all of those items is quite high for the age of the product ($70).
I bought Civ V + expansions through a Humble Bundle[0], probably for around 10 USD - 15 USD. In the past there have been at least 2x a Civ bundle and I'm sure there will be more in the future.
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