To start, being a courier is not about what kind of bike you ride. I know guys that kick *** on the street, and ride ten dollar sally-ann bikes. They don't lock them up, and if they get stolen they just go get another one. Lots of couriers leave their best bike at home anyway, and ride a second one for the streets where bikes, and gear see more mileage, and abuse than warranties are written for. Fixies (track bikes) are popular for a number of reasons, one of them being that there are less parts that need fixing. 'No brakes. No gears. No problems.'
In my opinion the most important piece of equipment for being a courier is your head. You have to read waves of traffic like a surfer reads the ocean. To make decent coin you have to really want to ride your *** off. You don't make money standin' on the corner trying to look cool or complaing to dispatch. To get respect: work hard, and keep your head up and aware.