This is highly generic as I do not have an invincimonk, though I used to trick out my E/Mo as one before the AoE update. Still, what you most likely want is Mo/W because of Balthazar's Spirit synergy with Bonetti's Defense. At high smiting, Spirit can easily power the high adrenaline cost of Bonetti's Defense which can then provide an alternative way of recovering energy. For cheap but functional items, I'd go with the 10%recharge collector axe with a +enchantment duration grip, to extend all of your non-maintained enchantments (Protective Spirit, Healing Breeze, Healing Hands, Shield of Judgment, etc., etc.) The easiest offhand to acquire is a 20/20 collector focus from the desert; the Smiting one offers an obvious advantage of cracking out already good damage enchantments into overlapping durations when combined with +enchantment duration weapon mod.
Generally, though, any kind of Mo/x or x/Mo should be able to use the Invincimonk build to farm, though it may take some longer than others. That's why they call it Invincimonk, you know

Since I haven't researched the topic, I think the only other interesting secondary would be Mo/N, to use the elite enchantment Aura of the Lich to reduce the already low health by half. Then again, if you're killing with Shield of Judgment, you may not have the elite slot to spare.