I'm going to assume you're a f2p and with the short time playing this game, I recommend you to * with the lineups in current meta before saving. It takes about roughly 6-7 months pulling only from kage treasure to get the super rare by using only the free scrolls you can get in game. If you can wait that long after the GNW release (in which you said you are capable of saving for 6 months), then * to the current meta since the GNW super rare pull range is roughly the same as the Kage treasure. Saving for GNW should be for those who already have roshi, han, 3rd raikage, and/or kage super rare but primarily roshi. Since you are planning on getting sailor soon, it's perfect since there's several team wtih her and roshi.

As for arena, you just started playing and it'll take some time for you to build initiative to be able to battle through arena more smoothly (above 10k total initiative is ideal). Like what Garv and Veelocity said, the rares in the GNW treasure won't make a huge impact. The only team with rares I've seen on the China server is the lightining Kushimaru and Jinpachi lineup but that requires you to have more initiative than the opponent similar to the water blitz team (mabui, mei, tenten).

To know where to get each ninjas: https://luckgamers.wordpress.com/2017/05/14/naruto-online-ninja-treasures/