If you play soraka right, you won't need many survivability items. Ghost and Flash should keep you alive during sticky situations.
Health quints aren't necessary but I use them for the laning phase, its nice to have that health pool in case of ganks.
Flat AP is also very nice for the beginning. Keeping your laning partner alive is very important.
Build order - I keep Amplifying Tomb for a while, I focus on boots first then grab Morellos. Its important to get a lot of CDR ASAP while also getting AP. You won't have to worry about mana regen if you level infuse like I do. Next I buy deathcap, after that it really doesn't matter. You could keep building more AP and get the hourglass or if you feel yourself getting focused, try aegis then Guardian.
Surviving on Soraka is very crucial. You don't need tanky items to do it, just your brain. Always play super cautiously. Run in as far as you need to, to heal. Run out. Repeat. If you sense a kat ult or something similar, keep yourself within distance to Silence. I wouldn't bother with starcall until you have 4 ranks of heal and infuse so you can better support your team. During the laning phase, bushes are your friends and last hitting anything is not your priority. All gold should go to your carry if hes around. If you are trying to gank, be ready to heal and silence the target for added damage. Still be wary, they can turn around and grab you instead if you over extend. Soraka's ult should be saved for life saving opportunities. If against teemo, pay attention if someone is near death and someone hits one. Karthus's ult can be completely shut down by Soraka if you put enough AP on it. It is always crucial to save your ult for when he does. Don't feed the karthus! Group fights are very intense for Soraka, don't keep yourself within reach of anyone. Irelia and other champs will find it easy to hunt you down. A good group will notice when you start running and try to shut her down.