There are ninjas that can only be obtained by purchasing ingots e.g. Itachi New Asian Style, Indra Susano
But there are powerful ninjas that can be obtained without purchasing ingots like Indra Otsusuki, Shisui Susano. Although f2p players may take several more time to get them just by coupons or events

Recharging isn't required to get powerful ninjas but it does make it faster

All of them have low rates, so it all comes down to your luck