I would stay one prayer until 65 or 70+ combat to be honest. But yeah i would say it would be worth it. Also, i would stay 50 attack just because jmaul rapes. But that is just me.
if you pure pk around lvl 70, only people you will be vsing are other pures and you will get bored as fuck very quickly.
around lvl 90 is okay but yeah still boring.
pking sucks atm you get PJed alot stupid d clawers who are sad just spec you and then teleport.
but still i enjoy pking on my lvl 81 combat pure ^^
so it is worth it i think and definitly get 50 attack for Gmaul ^^