With this build you start with 20% cdr. This is useful since you can heal your adc more frequently,poke with your Q and don't use your E unless absolutely necessary. Since you have mana and health regen runes and masteries you have an advantage. It will take you a lot longer to run low on health and mana.

This build doesn't have any ap items and that is completely fine. I find the sacrifice of ap to be able to heal your allies more frequently and without putting as much strain on yourself as soraka. Health over ability power.

With this build you have a total of 525% health regen and 3305 health at level 18 full build (at the very least). This is not including warmongs heart and gargoyle stoneplate's active. Warmongs heart heals you 25% of your max health every 5 seconds if damage hasnt been taken within 6 seconds. Also there is gargoyle stoneplate's active which increases your health by 40%, 100% is 3 or more enemies are nearby.

You'll have a total of 1371 mana at level 18 and you'll have 150% mana regen. That may not seem like much but add in that talisman occasionally drops a coin of 15% of your maximum mana. This is also why you'll have mana regen runes along with masteries.

I have found that Soraka plays well against Leona and Thresh. The reason is whenever they go in you just use your E beneath them and suddenly they can't do anything. Then they realize they cant engage without getting caught and outplayed. Which is trouble for them since they're supports known to have good engages and if they cant do what they are known for then they are pretty much useless. Soraka isn't known for her poke potential but her Q and E have quite the range. I have found that I can easily outpoke a Sona, Janna, Braum, Bard, and Taric. It's surprising but with good positioning you can do very well against these champions.

Soraka's base magic resist and armor at level 18 is 88 and 38.5. With a full build you get an additional 85 armor and 95 magic resist. If you add in the passive of gargoyle stoneplate you'll get an additional 40 armor and magic resist if 3 or more enemy champions are nearby). So in the end you'll have a total of 223 armor and 173 magic resist (not including masteries or runes).

Soraka's Q heals her if it hits an enemy champion. Every half a second she heals herself 7/8/9/10/11. Your W heals 80/110/140/170/200 health. Your R is a global heal that effects all your allies, heals 150/250/350 (and if your allies are below 40% health the heal is increased by 50%. Your heals on yourself are increased by 58% and heals on allies are increased by 20%. On a last note, heals and shield power is 3 times as effective on redemtion's heal.

To break it down, with full build and level 18 you'll have:
Health: 3305
Health Regen: 525%
Mana: 1371
Mana Regen: 150%
Range: 550
Attack Damage: 101
Attack Speed: 0.625 (36.4%)
Armor: 173
Magic Resist: 133
(excluding passives, masteries and runes)
Movement Speed: 325 (+70 +23%)