Best set up for an act 1 mercenary

Helm
#1- Giant Skull- The Knock Back Helm, perfect for warding off approaching enemies. Can have up to 2 sockets, 35 strength, 320 defense and 10% Crushing Blow.
The 2 sockets can be used to increase the survivability of the mercenary - 15ias/15@, or 15ias/40%ed. 15/15@ will allow to break the IAS fpa breakpoint with a level 15 Fanaticism Faith made in a Great Bow, and add a nice stack of 30@ res. 15/40's will offer extra damage instead.
#2- Andariel's Visage- 20% Increased Attack Speed, 8-10% Life Stolen Per Hit, +25-30 To Strength, +10% To Maximum Poison Resist, Fire Resist -30%, Poison Resist +70%.
Socket with 15/15@ or 15/40.
#3- Crown of Ages- If you can meet the strength requirement, this is quite the best. Main attributes: Damage Reduced By 10-15% (varies), All Resistances +20-30 (varies), +30% Faster Hit Recovery, Socketed (1-2) (varies). Socket with -15req/15@ to lower the high strength requirement.

Armor
#1- Treachery Runeword- You will be seeing a lot of builds with this armor as the highest recommended one. Why? 5% Chance to Cast slvl 15 Fade when Struck, +45% Increased Attack Speed, +20% Faster Hit Recovery, Resist Cold +30%. This armor is perfect for almost any situation. When the merc is faded, it has +60@ resistance and 15% Damage Reduction. What makes it even more superior than Chains of Honor (offering constant 65@ res) is the 45% IAS, the 20% FHR and the 30% CR (when faded, overall %CR is 90% resistance on the armor alone!!)
#2- Chains of Honor Runeword- As a supporting role, all you will need to worry about is the continuing survivability of your merc. This armor offers quite a bit extra damage to boot. +200% Damage to Demons, +100% Damage to Undead, 8% Life Stolen Per Hit, Replenish Life +7, All Resistances +65, Damage Reduced by 8%.
#3- Shaftstop- Unique Mesh Armor. Upgrading a good %ed ethereal one will produce with a crazy amount of defense. +180-220% Enhanced Defense (varies), Damage Reduced By 30%- highest %DR armor in the game. +250 Defense Vs. Missile, +60 To Life.Socket with a 15/15@ or Um or even a Cham if you don't want your mercenary to lose speed while wearing Giant Skull and providing with many KB hits per minute.
#4- Skin of the Flayed One- This extremely underrated unique Demonhide Armor is one of my top favorite armors of all times. It simply has everything a mercenary needs to survive. +150-190% Enhanced Defense (varies) - massive defense while ethereal. Repairs 1 Durability In 10 Seconds- doesn't matter since it's on a mercenary, but it goes to show it can be used as ethereal on yourself as well! Replenish Life +15-25 (varies) - highest life replenishing armor in the game. Attacker Takes Damage of 15. Socket recommendation as said above.
#5- Lionheart Runeword- A very old and beloved runeword. Quite cheap to make as well. All Resistances +30%, +50 to Life, +20 to Vitality, +15 to Dexterity, +25 to Strength. There are several more mods, but they are irrelevant here. This armor is basically a tank, adding up to 30@ resistances and 90 HP, not to mention dexterity and strength.
#6- Spirit Forge- Unique Linked Mail. +120-160% Enhanced Defense (varies), + (1.25 Per Character Level) 1-123 Life (Based On Character Level), Fire Resist +5%, Adds 20-65 Fire Damage, +15 To Strength, Gem Sockets (2). Socket preference as said above.