The 3 you have mentioned are worth it - especially Carol and Medusa. Also have in mind that even if you spend 120 on Pun or Nova (both very good and fun to play) you will not be getting many champion levels for them (probably like 2-3 a year).
>The pain of transition is you get covers for the ones you don't care about. Write them off as part of the game (aka 1k iso) or champion them all. I suggest you keep opening the classics so you can cover the top dogs you mentioned before they rotate out. No point waiting.