I run a lot of addons but for a healer I highly recommend squared. I have every player in my warband in a little 2x4.5 box between my combat log and chat log right under my characters feet, colorcoded according to class type RDPS, Healer, MDPS, Tank, along with symbols further defining the character class. It makes it very easy to target anyone for quick healing. I also recommend using CoolDownTimer and PulseIcons so that you have an eye out for your cooldown spells. It flashes a LARGE icon up, letting me know when my ability has come off of cooldown and is ready for use, along with a big flashy text on your screen (this is pulse - it also updates you for a melee character on things such as Parry or Block so you can use your abilities that require those as triggers). The cooldown bars give you a timer on all cooldowns currently running.
Really you don't NEED any addons or mods, but life is SO much easier with Squared when you are in the middle of a heavy combat situation and really don't have time to let your eyes stray from immediate surroudings. It allows for resizing, texturing, fill direction, AP percentage, color changes based on player health and all sorts of other goodies (tank starts fading yellow/red - heal em! that bw just went from full to 1/4 health? RUN AWAY ITS A WITCHELF)
Autodismount is a blessing for riding up, targeting someone (friendly or enemy) and spamming whatever abilities you normally would and allowing you to bypass that whole r-click to dismiss buff, or using the Summon Horse again to pop off of it.
TargetRings is also nice, but that's really getting overboard if you are a minimalist. Most of the things found on Curse are safe, and if you are worried about them you can actually read the source code. There's a few of us in the guild that can dive into it and give you a heads up on any out of place, or potentially harmful code. I've found nothing even remotely malicious there, so you should be safe with the addons found on Curse.
Good luck with the healer!