I don't know how I missed it. I sat on the page and refreshed for 15 minutes. : (
That's quite some dedication there.
Then there's email DLC for each idol.
Oh, yeah. I forgot to mention about those. As Okayu mentioned, make sure you buy these immediately if you want to start seeing texts messages from the idols when you start the game . (Not required, but an interesting feature to have).
But, in any case, if you like the game a lot, then there's nothing wrong in spending extra money on some DLC. If you don't like the game at all, then there's no point in getting DLC. Obviously, you don't have to get everything. Just pick out what you like or can afford.
I know some people will disagree with my philosophy, but I feel you'll miss out on parts of what the game offers if you 100% avoid DLC. Getting some DLC is good enough. And, you don't have to buy it immediately as it comes out. You can buy whenever you save some extra cash. As I mentioned before, it's not required, and you can always see people upload vids for songs and story DLC as Okayu mentioned if you can't afford it. But, if you have extra money and you really love the franchise, then by all means buy some DLC you like. You'll enjoy the game more. Or you could wait a year for the bargain.