A Doom ethereal cryptic axe is a great choice as well, since it has huge damage, gives your merc +2 to his aura, and gives him the holy freeze aura as well.
Helm-wise, a Ral'd Andariel's Visage is pretty much the best helm you can get for a merc, since it has +2 skills, huge defense, life leech, 20% ias, +strength for more damage, and massive poison resist, which is generally a merc's biggest threat. The Ral rune will remove the negative resist penalty. Ethereal one would be godly.
-Hal