All sets are in no particular preferential order.

Monster Set
Nightflame
(1 items) Adds 1064 Max Health
(2 items) When you heal a friendly target, you have a 3% chance to summon a totem for 6 seconds that heals you and your allies in area for 3483 Health every 1 second. This effect can occur once every 6 seconds.

Sentinel of Rkugamz
(1 items) Adds 2% Healing Done
(2 items) When you heal a friendly target, you have a 10% chance to summon a dwemer spider that heals for 1049 Health and restores 524 Stamina to you and your allies within 5 meters every 1 second for 8 seconds. This effect can occur once every 8 seconds.

5 Pieces
Healer's Habit
Dragonstar Arena
(2 items) Adds 967 Max Magicka
(3 items) Adds 129 Magicka Recovery
(4 items) Adds 129 Spell Damage
(5 items) Gain Minor Mending, increasing healing done by 8%

4 or 5 Pieces
Healing Mage
Aetherian Archive
(2 items) Adds 129 Magicka Recovery
(3 items) Adds 967 Max Magicka
(4 items) Adds 2% Healing Done
(5 items) When you use an area of effect heal ability, you reduce the Weapon Damage of all enemies within 10 meters of you by 430 for 3 seconds.

Robes of Transmutation
Cyrodiil
(2 Items) Adds 129 Magicka Recovery
(3 Items) Adds 129 Magicka Recovery
(4 Items) Adds 688 Spell Critical
(5 Items) When you heal an ally with a healing over time effect, grant them 1259 resistance to critical hits for 20 seconds.

4 Pieces
Mother's Sorrow
Deshaan
(2 items) Adds 967 Max Magicka
(3 items) Adds 688 Spell Critical
(4 items) Adds 688 Spell Critical
Bittergreen's Scale of Mother's Sorrow
Bittergreen the Wild, Caravan Crest
Kagouti Slayer Medallion of Mother's Sorrow
Short-Tusk, Short-Tusk's Hillock

Even at low levels, this build is taking multiple opponents to kill :) It's still mostly a theorycraft (only tested to level 19), but I'm liking it so far. I can keep myself and others up during a siege, either defending or attacking (multiple oil attacks to the face and I'm still power healing through it, it's fantastic). As a healer with almost 0 DPS, gankers are annoying cuz you can't kill them, but you can stay up damn well. Healing Thicket is incredibly powerful for how cheap it is; from the time an outpost's doors go down to the time the flags flip, I can fire it off 3 or 4 times. Early levels, I'm using 5 Seducers with 4 Robes of Transmutation in PvP.