Strongest brushless setup for revo?

[Over_revO]

Ok, i convinced my friend to go more conservative on its build LOL


He accept to run a Neu 1515 or 1521 , but i will need your help guys.

Wich motor is better ? 1515 1.5d ,y , or 1521?? im lost with all those numbers lol

And if someone can suggest the best batteries to get with the setup you suggest,
the strongest batterie possible (6S)

Thx a lot for any help :)