Where do u play? normal?
But imo melee mercs are better than ranged. They give more damage and function as effective 'meatshields' like u said. Especially when u walk around, u dont want a defensive merc that is always behind u
If so i would suggest a2 prayer merc (defiance and blessed aim isnt relevant)
There are nice polearms/spears around like: Battlebranch, Woestave(50%slower enemies!), The Tannr Gorerod(126max damage lvl 27

), Bloodthief (12ll + 35% open wounds). So there are quit some possiblities. But mainly ur merc wants life leech. Amn rune in weapon would make a big difference
For helm i would use a undeadcrown (5ll, psnres50%, 50%dmg undead, half freeze duration and splendid defense)
Coif of Glory is also nice for the 'hit blinds target' mod
Normal armors are very interesting, there are some very good ones around:
Rattlecage: 25% Chance of Crushing Blow, Hit Causes Monster To Flee 40%
Rockfleece: Damage Reduced By 10%, +5 To Strength, Damage Reduced By 5
Boneflesh: 5% Life Stolen Per Hit, 25% Chance Of Open Wounds
Hawkmail: 10% Faster Run/Walk, 15% To Max Cold Resist, Cold Resist +15%, Cannot Be Frozen
Twitchthroe: 20% Faster Hit Recovery, 20% Increased Attack Speed, +10 To Strength
Greyform: 5% Life Stolen Per Hit, Magic Damage Reduced By 3, Cold Resist 20%, Fire Resist 20%
Whatever u choose, eth means more damage/defense so thats better, try to aim for that
A5 mercs are good aswel, some swords are nice. One with decent damage, ll, and other mods like OW, CB etc are always good. And the same counts for these as for A2 mercs. More lifeleech means a better chance of survival. I would recommend a 'stun' a5merc.
Just see what suits u best, a combination of a a2 merc and a barb merc is good as they will definently protect ur asses and throw themselves into combat. 2 a2 mercs are also fine, but if u always play together, there isnt need for another prayer aura, and the other aura's are pretty useless for casters