I feel there should be no paywall in order to customize a hero. It makes the choices of heroes negligible as you won't want to pick another one when you can upgrade your main instead. I could get an elite box for the same cost
Customization has fairly negligible effects at low renown levels and all heroes are playable without "recruiting" them. Also the cost to recruit is VERY low considering the steel gain in the current build. You can get enough to unlock a new hero for customization within about an hour to two hours of dominion matches easily.