Start with Meki Pendant. Upgrade to Fiendish Codex.
Now pick up some boots.
Why don't you get mana manipulator early on? Because it's not needed that early game. The aura effect is useful when the game starts to approach the later stages, where grouping up for extended periods of time take place.
So after these two items, then you can start working on Giants belt first and finally Soul Shroud. You'll have pretty good mana regen for yourself along with 25% ability reduction speed.
I usually then go Rylai's Scepter and or Aegis of the Legion, the order is up to the situation at hand.
You'll be pretty tanky with a high HP pool, 25% ability reduction speed, and a nice pool of AP.
Rylai's I like on AP soraka.
@How you afford - You usually don't. But if oyu manage to get every item, that's what you would go with. I usually end games with Locket/Treads/Aegis and maybe a giant's belt towards soul shroud.