I agree with the person above...
Lightning seems to be really nice
and Fire though I haven't had too much time to play with seems pretty good to start off with as well.
Azure is decent but Fire and Lightning seem to be better to start with.
P2W in the sense that if you want a certain ninja the moment it is released...you can see yourself spending more than just $50-$60 for lol. I suggest buying ingots and trading them for coupons under the Hot Topics icon as well as just saving coupons you get ingame...coupons and ingots go at 1:1 basically worth the same.
Ninjas keep being released through events again and again though...so if you don't want to toss out $100 an event or so...you c*ways just go with the events up until you feel like you want to buy one.