There's only so many champions. Select 8 or whatever number you decide, and name it Rotation 1.
Select a good medium between 450 ip and 6300, rotate evenly or try to anyways. But also have some knowledge between the characters that are going to be free, and try to make it good competition. Like if you decide to make shen free, throw in a vayne, Or just completely random it, -2 and throw in the easy champs. Then you might have 3-6 6300 champs. But only because there's so many.
If you're worried about new guys now having an easy champion, choose your "easy" champions, make a list, and divide them evenly among them. I think your version is the lowest costing. (450 ip or w.e)
In my opinion, if you read their skills before choosing the character, then you're probably going to play for a while. If you jump right in to the game with no knowledge of any kind, you're up a creek. And I have your paddle.