All DLC content is included in base game, but hidden if you haven't bought it. Playing with different DLC combinations is not an issue, as you will be able to see content from other DLC's, but only buy the ones you own. The European cities DLC seems to be an exception to this, and be required to even see the buildings.
I do have to warn you thogh that the multiplayer has an extremely frustrating desync bug, and also that theres very few if any public games.