Well, considering the only ninja you are not able to get as a pure F2P person is Cheongsam TenTen, your options are varied (considering I have a F2P account that has spent all its coupons towards Edo Tobirama and I have 50/80 frags). Considering you didnt give what ninjas you have, its hard to say what main you should go for. The easiest to build F2P teams around are Wind and Water. As for Treasures, you can't go wrong pulling in GNW treasure as a way to get closer to your Kurama Naruto or Madara, but if you want a few complete lineups before that, Jinchuriki I and II as well as the Land of Lightning Treasures can offer you a full team for ~180 seal scrolls.

As for event ninjas to go for: Shisui, Edo Deidera, Sailor Sakura (still quite good), Kisame Fusion Mode, and Kushina Habenero are some to focus on.