I start with ancient coin because of the extra healing it gives me over Spellthief's edge. Like spellthief's edge, letting the item pay for itself and then selling the item back instead of upgrading it still works out in a net gain.
I of course rush sightstone because it's mandatory on supports, and then I go for either Morellonomicon or Athene's unholy grail. Soraka needs the mana to spam her Q to heal herself so that she can heal others. If you want to finish Talisman (which isn't a bad choice at all) you can forego the mana item since all three do the same job. Athene's is generally the better choice because of more mana, but if you're up against a team that does a bit of healing, Morello's can help close out kills.
I then go for either Zhonya's Hourglass or Banner of Command. Banner of Command is the better option, what with the cdr, regen, and how the aura can support others tanking for her, but if most of the enemy team is AD, I'll go for Zhonya's, since Soraka is the only support that gets focused. If you want Talisman, I recommend picking up both.
After that I get a deathcap because more AP=more heals, both self and ally.
Finally, If the game is still going by then, I pick up a Guardian Angel, because it is essentially the only way to build more health on Soraka without making her heal cost more.
This is merely my recommendation of items for Soraka.
For you, something less extreme seems to be in order, and so Deathcap could be out of picture; not enough AP items. I disagree with Spirit Visage and Warmog's. The passive isn't really worth it compared to AP.
Let's consider what Spirit Visage does for you:
+400 Health
+60 magic resist
+150% base hp regen
+10% cdr
All self-healing effects +20%
Here's Soraka's self-heals:
「 MINIMUM SELF-HEAL: 25 / 35 / 45 / 55 / 65 (+ 40% AP) 」
「 MAXIMUM SELF-HEAL: 50 / 70 / 90 / 110 / 130 (+ 80% AP) 」
At 0 AP Spirit Visage will increase your self-healing by a minimum of 13 and a maximum of 26 each time. (max rank only)
If you build a Banner of Command, it will increase your self-healing by a minimum of 24 and a maximum of 48 each time. (any rank)
Since the cost is % health, each time Soraka uses her heal with Spirit Visage, it will cost 40 more HP.
Each time Soraka uses her heal with a Banner of Command, it will cost 20 more HP, but also heal 36 more from the AP.
While Spirit Visage offers passive health regen, building HP in any form is pretty bad on Soraka.
Warmog's is in a similar boat. It will cause Soraka's heal to cost 80 more HP.
As such, if you want magic resist, I recommend Athene's and/or Banner of Command.
To recap, these are what Soraka wants, discluding your choices of Visage and Warmog's:
Mandatory:

Mana items: (pick one)

If Talisman: (more cdr is better)


If Athene's/Morello's: (pick one)
+An extra item in the "pick two" below
+(Your choice of boots)+10% cdr masteries
And then pick two:
(Only if you already have 2 AP items)
(Only if you don't have 2 AP items yet or if you already got Rabadon's)
Other items:
Not a bad choice per say, but Soraka can't fill up a tear very fast, and she can't take advantage of the Seraph's+Rod of Ages combo because of hating health. It also makes you give up maximum cdr usually.
It's possible to make it work, but you need AP so that the passive can boost that as well, otherwise it just hurts you. It also makes cdr juggling a bit harder.
It can only apply to your Q, which already has a slow if you land it dead-center. Has health, which is poison. Overall pretty meh.
Raw AP basically. It's not a bad choice at all, but I can't fathom why one would want to try outputting damage as Soraka instead of other effects to help Soraka stay alive.
Not a bad choice either, but it scales off of health, which you are allergic to.