Soraka is an awesome bot. I posted this a couple days ago and it's how I play her:
It's all about picking items that make you stronger and counter your aggressor. That also goes with skill leveling. I firmly believe that Soraka shouldn't level her skills the same way every time. For instance, against yorick, I would max starcall first to destroy his creeps, then silence then heal. As for items, I would start with a catalyst and work into rylais next. After that something like frozen heart or thornmail. The rylais is really needed vs champs like udyr and yorick as your silence will make kiting them fairly simple. The FH or thornmail will help you in case you can't constantly kite them. Once they fully commit to fighting, I use ignite if their lifesteal is out of hand.
Sivir and Urgot and pretty much other ranged dps champs might make you have to put a few more points into your heal to stay in lane and more focus on silence for urgot and starcall for sivir (since sivir's push can get out of control while urgot relies on his low cd nuke to be used on you).
I either run mpen (reds), cdr(blue/quints), and armor(yellow) runes or I run mpen with AR/MR yellow/blues and hp/5 quints depending on whom I'm going up against.
Oh, I also just went on a soraka/ww spree on my alt and won 9/10 games with soraka and 8/8 with ww on my last 18 games played
Also, another thing to win lanes against most of those champs is to run around randomly spamming as much as you can. Never run directly towards a champ because then your moves are predictable. Always just run randomly back forth up and down and they will miss their skill shots.
Don't forget to not over-extend. Soraka is an amazing pusher and thus you can choose exactly where you want the creep wave to stay at, even against amazing pushers like sivir and heimer. This can also help you win vs champs like kass.
For items, she really is versatile but like said before, tanky ap items and tanky CDR items are beautiful for her (SV, FH, Rylais, Thornmail, AbSc, Odins, and even lichebane). I love thornmail and odin's veil especially since they do magic damage and can be amazing damage when you have shredded their MR and Odins can do some awesome burst that people forget about.