depends on how much money u wanna make. most of the high-stake matches are from maxed combat players. otherwise, 90 90 90 with 1 pray/sum would be an ok place to stake. you'll be around 103 combat and could box lvl 120s with the same stats. possibly make them double you.
I've only ever boxed with a maxed char. As I Inimical I said though, you don't need summoning or prayer for boxing, so if you leave those at one you will be able to box anyone at a much lower combat then them.