If I had to pinpoint a power range when you should start thinking of making a team with event ninjas, it should be around high 60k - early 70k. This really depends on how well you are in pvp with the f2p metas. You also have to consider how much coupons you normally can farm.

My advice is to slowly piece ninja via free points and use coupons on power. By the time you are ready to get an event ninja, you c*ready have most of the frags. Just make sure what current ninjas can be used in the future (you do this by watching youtube videos on the China server to see their metas). This advice applies to any other games where there's an much older server released before the one you are playing on.