How do I get the most out of Fiddlesticks?

I don't feel comfortable as a jungler to play him there, so I generally play him mid or top, depending on the situation. I usually go Doran's Ring, Sorcerer's Boots, Hextech Revolver, RoA, then Rabadon's or Zholya's. I have heard that Will of the Ancient is good on Fiddle, but I don't usually get it. My pertinent questions are:

• What is spell vamp, what does it do, and do I need it on Fidd?

• How do I maximize my damage in team fights? I know that Crowstorm is a game changing ult, and I feel that I'm beginning to develop a decent ability to time it, but how do I use his Q,W, and E to maximum effectiveness?

• Should I build straight AP, or should I build armor and things as well? I know the situation generally determines this questions, but I'm looking for general tips.