Life Steal vs. Spell Vamp

(1) The item I was looking at was the Will of the Ancients (+50 AP with an aura giving +30 AP & +20% spell vamp). I was thinking that it would give me a decent bit of AP to make all of my spells stronger, and also provide a bit of healing for myself and teammates.

(2) Currently, I'm starting with Boots of Speed + 3x HP pot. Then getting 1-2 Doran's Rings for the mana regen (Blue buff works wonders for Despair), followed by sunfire, Abyssal Scepter, and Frozen Heart. I love auras... can't you tell? haha.

(1) Tankier champions tend to take auras such as Frozen Heart or Aegis, which are best kept on a member that's going to (hopefully) survive the longest. Will of the Ancients is best for your AP carry because they'll want the Spell Vamp aura on them so they're no reliant on you. Not to mention the Spell Vamp aura isn't going to be as important for you as it will be them.

(2) It's important to be flexible in your builds. Starting items can be a little more rigid since there's only a few combinations that are worthwhile. If an AD carry is getting extremely fed, try shutting them down with more armor through an item like Thornmail or Randuin's Omen (or Frozen Heart. I think everyone goes Frozen Heart now). If an AP carry is getting fed, try shutting them down through MR. You need to analyze who is doing well and then buy items accordingly. That being said, don't forget to pay attention to when (and if) the enemy team has a shift in someone getting fed. It's no good to stock up on armor to just down the ADC when the ADC is now already shut down with your 100 armor and the AP carry is now getting fed and you have no MR.