Best merc is Act 2 NM Defensive Merc for the Holy Freeze aura (which will help slow monsters for your hammers to hit). Ideally he should be given a elite insight polearm for the meditation aura and large dmg. Throw on a fort for the extra dmg and put an um'ed guillaimes face for crushing blow against bosses.
I agree with bogie. I'll go further to say that Insight damage, while nice, is not very important for a hdin. If you dont have an elite pole arm at the moment, make an insight in the first 4os pole arm you find (regardless of ilvl). That way, when you get a nicer insight, you can replace it, but still have a low lvl one for developement of chars in the future. Aside from insight, merc equip is pretty arbitrary. Fort and Guillaumes are both excellent, but you hardly need them. Put on any helm with life leech, and any armour, and you should be fine.
You'll be doing most of the killing, but thats what hammerdins are made to do, arent they?
However, my ideal set up would be:
Eth CV insight
Eth Shaftstop (with ber or um or pruby)
Eth Vamp Gaze (maybe with a cham, or a ber, um or pruby)
Actually i think that Insight is not really good for a pally mage (i.e. hammerdins)
This is because you should have put 1 point in redemption (an offensive aura), the 1 point wonder skill which gives you back mana/life per corpse.
I'd think a reaper's will serve you well because of its chance of casting decrep(sp?) and also the life steal to keep your merc healthy.