There are other considerations as well when it comes to not going Oom in a fight, one should possibly consider downranking of spells. Even if you have the best gear you could possibly have it is possible to run out of mana. By downranking one or two ranks of the spell you spam the most you will use less mana and have more time at casting. This also means more time for in cast regeneration. The loss of damage should be minimal on a per cast basis and since you are casting for longer the amount of damage that you do has the potential to end up being higher in the end.
As for the "best" spell to spam, I find that Starfire is by far the better choice. Just using MoW I have about +103 mana per 5. So for about every five casts of Starfire at max rank I regen enough mana to cast the 6th one. By that same token it would take the time spent casting 10 Wrath spells to have enough mana regenerated for the 11th. Over the course of the castings the wrath spammer has used up about 2.3k mana at the least. I using starfire have used up around 1.7k mana. This is of course not taking crits into acount for reduced cast time. IMO the add DPS of the Wrath spell (if it truely does more damage) is not worth the loss of longevity in a fight.
From the numbers I usually see my wrath ends up doing slightly better than half the damage from my starfire right now. (Wraths hit for 950 damage on average while starfire is about 1750)
As for gear
Mandragal is outfitted in gear that can be found prior to doing any Karazhan raiding except for one of the rings which drops in the chess event and the T4 gloves. The hardest things to come by are the Epic crafted pieces (Windhawk set and the Windscale Hood). The rest of the items are either dungeon drops from non-heroics, killing the horseman, and rep rewards.
This was a little OT from the origional post, sorry for the tangent. As for the weapon I would use while regening mana, I would go for the Dreamer's dragon staff for end game. The trick is getting a lucky drop off the Satyr boss in Botanica. Stick with a caster item for while you can stay at range then if needed switch to the staff to maximize the regen you see in melee.