all you need to ride immediately is a new wheel, and an iro is probably the best bang for your buck. you're going to be good with that for a while... just long enough to get hooked on fixed wheel. after that be prepared to shell out money for new cranks, chain rings, cogs and all manner of rolled up jeans and messenger bags.

also, a word of wisdom: if you plan on converting your only ride, you may want to pick up a cheap beater frame on ebay or craigslist so you can learn on that and mess that up all you want, then when you get more confident convert your primary bike... you'll probably have stopped using it long before you convert it. i rode on a schwinn continental for about eight months before i converted my favorite bike. i didn't want to get stuck having to do a whole lot of cable routing if it turned out fixed just wasn't for me.