I like some of you ideas but have a few suggestions:
Rod of Ages is cheaper then Rylai's and gives better health, identical AP, and mana. The slow on rylai's will only be useful on infuse anyways because it is reduced to 15% on starcall
Why not build the fiendish codex into a DFG or MET( Morello's Evil Tome)? Then you get better CDR and AP...DFG is great against tanks, its active just gets rid of at least 30% of the target's health. MET gives better AP but is cheaper and has no active...I get both when I play Soraka...
I also have never found Soraka to be a bad solo lane or mid. She is my favorite mid out of all the champions I play.

Noxxeh: I like Shurelya's Reverie, but I feel it's just not for this build. It is a good item, but most of her items center around AP, not necessarily cooldown.

Jebus McAzn: I'm about to edit to add more info, but really the item progressions are simply to show smaller items to bigger items and should really be purchased in the order needed.

DiscoGrind: Starcall is a good farming spell, but I feel its use detracts from her primary role. Your lane buddy should be doing plenty good farming. Soraka's there to keep them alive in fights.

U cant play Soraka without Stacall, awesome farming spell and very usefull in teamfight(-8 MR.. stack 20 times)

Spirit visage must be one of the first items

spellvamp is useless, u doing very small damage

I'd honestly suggest the Spirit Visage earlier since it makes your heals so incredible at all phases of the game - it would help your laning phase, since Visage is a relatively cheap item.

Looks good, would like to see it maybe be a bit longer and how to play her early, mid, and late game. Bu really a pretty good guide, well done.

I suggest adding that one flaming crown looking thingy; that crown saved me and my remaining team mates from a lost team battle countless times. The speed boost is also helpful on top of ghost to get out of sticky situations or just to get a boost to the battle front.

P.S. +1 but some minor changes would be nice.

Ack! How could I forget! I new I was only up to five items, but I couldn't figure out what I was missing. Updated, and thanks. :)

No Spirit Visage? I find that item to be absolutely essential on Soraka.