I think an Act 2 merc would be best... Becouse his Aura can help you..
Holy Freze: Help by slowing all enemys (cold Imunes too)
Blessed Aim: Boost your ar.
Might merc: Boost your dmg.
Now it up to you what you want/need..
As for weapon something with Leech and good dmg will do.. Could be Pride (Ladder only runeword that give Concentrate (boost your dmg too)), Bonehew (Cheap, socket with 2 amns or Amn sheal) or Botd Wp (good dmg, leech and stuff.).. Now there is alot of other good weapons for act mercs, this is just some ,.... (remember that eth items is better cus they dont lose Durability on a merc..)
For helm: Andariels visage is getting more and more used, it add str, speed, leech and some more stuff, Gaze add Dr% and leech and its fairly cheap, tal helm, add leech and res.. Again, more helms here this is ´just a few of them..
Armor: Shaft stop add alot of dmg reduce, Coh add very good res, 20 str and good leech, Fort add dmg, res and life and its Ladder only that can make it... Again, more armors this is just some i could think of ...
Hope it helpen a little..
~Shadow