I would advise against getting a ****ton of characters.
I would first make 6 champions, then make sure that they are done and complete (and balanced). Before adding anymore champions.
Which in my opinion means; more items, skills, levelling, perhaps mana bar?
As for champion's and buying, I would make 3 free per X amount of time, then make another 3 free instead. This way people can still get to play as all champions, but have to buy one if they want to be sure that they can play the guy at all times. (A bit like the bloodline system)
Note: This is just what I would do, just dropping it out as a possible idea, not an enforcement. :O