I get that exclusive ninja are essential for any end game team. But what about mid game?

My server is about 3 months old. I have 47 bp. I've been saving coupons for 5-6 weeks, but I'm unsure if I should buy a ninja with them, or is it too early? Would it be wiser to spend them on power and instead collect ninja fragments from events slowly? I need your advice senpais. Please share some of your wisdom and experience

Bonus question: what is the best way to increase power? Myoboku, charms, or battle armor/clothing? Or should I just spend on Lucky Board for whatever power items it gives?