I personally would think that they should add a button on pbe that will let you buy all champions and skins so PBE users don't have to go through the pain of wasting 30 minutes of their lives trying to buy all skins and champions then getting reset every season.
There are four champion bundles that you can buy (each for a certain portion of the alphabet, e.g. champions from A-F) that will give you all the champions.
You also don't need to buy every skin right away. If you lock in someone in champion select, you can buy the skin you want to play right then and there. There's no need to waste time methodically buying every single skin, since you probably only have one or two per champion that interest you and you can buy them when the time comes.
