So, I just rented RB2, and on one of the sets it tells me I have to have some other songs in order to play this one particular set. Now, here is the question I have: Do I have to buy those songs in order to beat this game, or what? I don't want to buy shitty songs for a game I've rented.

You have to play story mode to unlock.

Well, I have been, and in story mode there is a set that says I don't have a particular song by the white stripes. I looked online in their song list of buyable shit, and that song I needed was in there.

Sometimes, you have to play other stages first before they are unlocked in the stages. Say, you need to go Paris, then only the songs are unlocked in London or something. Something like that.