You are making it very unclear if you plant to implement a serious PvP system with rankings and points and rewards.
Fairness is not what breaks dueling. When a battle is unfair, you simple opt-out.
Diablo 2 dueling was not fair, ever, but it was the sole reason I kept coming back.
With the variability in every class, and the different dynamics of every skill, you are never, ever going to make a PvP system that is "fair" for all classes. That should not be your goal.
When the PvP is well established, players will know that no one wants to fight someone with perma-freeze equipment.
Just have the PvP dialogue [accept/decline] show what you're up against - you see what equipment is being used, and you can decide - is this going to be fair?
Then you fight.
You lose, you either learn and don't fight again, you learn and you try a new technique, or you don't learn and let people beat the crap out of you.
Its really not a hard concept. Just let us play!