Equipped creature gets +2/+2.
As long as equipped creature is legendary, it has hexproof. (It can't be the target of spells or abilities your opponents control.)
It’s possible for a Champion’s Helm you control to be attached to a creature an opponent controls (if an opponent gains control of your equipped creature, for example). In this case, the creature couldn’t be the target of spells or abilities you control, as you are an opponent of the controller of the creature with hexproof.