You don't want that type of merc.
Yes, you will use Act 1 (Cold Bow) merc.
Helm: Delirium -> With that helm if get more faster hit recovery, Debuffs VS monsters such as on the merc's hits: 11% lvl 18 Confugion. Not to forget the getting hit part: 6% lvl 14 Mind blast / 14% lvl 13 Terror / 1% lvl 50 Delirum (Once thats active it will bouble the attack speed of your merc).
Armor: Treachery -> Gives 30% Cold res, makes it have 75 in hell. 5% lvl 15 Fade, reduces dmg taken by 25% and grands max res @all. 25% lvl 15 venom, thats an extra 500dmg. 45% Increase attack speed. 20% Hit recovery.
Weapon: Ice -> on Stuck 25% lvl 22 Frost nova. lvl 18 Freeze Aura (it will freeze very thing, also Cold immune's). 20% Increase attack speed. +20-30 Cold dmg / -20 Cold resistance. Ignores Armor.
I guess this merc will easly give you much and much more survival ability's compare to the one that has been noted in here.
My self, I also use the near same Pala build. Only I also do baal, and those souls are really a bitch with the lightning attacks. Instead of 2x bk rings I use one with a Wisp for some Lightning absorb.
Phoenix helps alot VS Travincal and Chaos, if you do those 2 for MF, get the phoenix.