if you want ninjas then you should really be thinking about spending money on the game - power can be farmed up for free (or relatively low money) whereas there are many good ninjas who are premium and are not going to be farmed in any sort of reasonable time frame.

There are many mid level teams made up of "free" ninjas who are quite proficient to hold you until you pick up a premium one, but if you do not have the power then your sage naruto or roshi will quickly end up getting killed by higher levels and feel like a bit of a wasted effort.