Although I haven't played the game in some time, my advice on ninjas, particularly on spending during rebates, is still relevant. While he is likely cheaper now, one of the relatively inexpensive event ninjas that I recommend, and can be of great help on a new server, is Shisui (Kotoamatsukami). He has a dependable passive barrier, reasonable survivability with his dodge, a strong mystery with targeted chaos, and works well with several early ninjas (as he is a sword ninja and Konoha ninja, making him a good fit for synergizing with Iruka's passive or Lightning main Root of Warrior stuff).