Prepare yourself for a wall of theorycrafting text from Erayzer:
I think I've figured out what would be the best gear, theoretically.
Dex and AC would be useless, since having high to hit% will beat AC easily anyway.
So, I think having a king's prefix should be sufficient, maybe even having two iron rings could do the job. But, King's is the best prefix for damage available thus making iron pretty much useless in to hit regards. That's assuming Wirt sells affixes up to 2*clvl (which I'm not sure about).
Strength is pretty bad too, since the amount of damage it gives, compared to how much life we can get on same item slots, which would make +vitality or +life better.
Taking all of this into account, I think the best items would be:
Sword: King's Bastard Sword of Gore OR King's Bastard Sword of the Heavens
Shield: Obsidian large Shield of the Tiger
Helm: Obsidian Great Helm of the Lion
*Armor: Awesome Studded Leather Armor of the Lion/osmosis?
Rings: Iron ring of Zest, Iron Ring of the Jaguar, Ravens/Jaguar or zest (For more healing and mage variants, we duel without potions)
*(might as well put AC here, since body armor is most probably giving more AC than anything else, and already having 2 Obsidian items will most likely give max resists).
The lack of Prefix choices makes it hard to come up with really good rings and armor.
-Raven's ring of the mind for mage-build.
-Obsidian helm and armor of sorcery for mage build again.
-Maybe all attributes instead of vitality for bow users.
-Adria was selling Bountiful staff of Phasing once, I thought this could be good also for mages, allowing unpredictable movement in duels.
-Maybe sword of sorcery, with dmg/to hit% for mage/warrior hybrids, but even then, I think King's heavens beat this.
-Blue/White/Red rings of +Life/Vita if resists aren't high enough
-Rings of the sky are not to be underestimated.
I think the most expensive item we'll have to buy is a King's Bastard Sword of the Heavens.
Based on the formula: C= Bp + Bs + Qp + Qs + I (Mp + Ms)
a perfect King's Bastard Sword of the Heavens would cost 112900. But that is Griswold's price.
For Wirt, multiply Griswold's price by 1.5. Sadly, it means we'll have to pay 169350 for a perfect King's / Heavens. So I would advise not shopping before having at least this amount of gold, which can be very inconvenient since so much gold takes nearly all the inventory space.
-Erayzer (with small edits by Legitdemowar)