Power will only beat p2w ninjas if you have like 15-20k bp advantage otherwise paid ninjas have extremely high growth rates compared to what's in the in-game shops. I myself breeze through ST on extreme difficulty because 90% of the teams there have 60-70k bp but they don't have many rare ninjas. That said however you can still make a good rare ninja team by just being f2p. There's many cheap good paid ninjas like sailor sakura Kushina and some commons from the new gnw treasure are very good too. If you have been playing for 6-7 months and don't have a single rare then you either haven't been doing wars or they aren't giving you enough packs. for 6-7 months if you started on a new server you can collect up to 300-350 seal scrolls total as f2p if you don't focus much on power (that's a ton of rare ninjas), after that you can power up while enjoying the game too even if you have low power bonds arena and gnw are always more balanced and you will have more fun there.

there are 2 konans, the one in china is a paid ninja I think.