Don't want to rain on your parade, but I doubt you'll be able to piecemeal it out like that. They've taken steps in the past to prevent such a thing. It's more than likely that the game will be on one disc, and expansion I, expansion II, and The Taken King will be on another. And I can't imagine anyone wanting to just buy the game alone when the other content is available for not much extra. But I wish you luck.