I second what the 2 others said.

It's better to farm out the pains in the shops. That being said, with 400 scrollsyou should get around 6 rare ninjas. You will probably get 3-4 different pains on average with 400 scrolls.

At that point you will get either Tendo or Hashirama, who drop at 400 each.

Honestly those ninjas, although classy and cool, are weaker than those you can get from jinchuuriki or kage treasures. Let alone the future treasures we will get. So you should probably save your scrolls.