This should always be the first question when buying any sort of bike, but triply so for a fixed gear. For starters, have you ever actually ridden a fixed gear before? Do you know how different it is from a "regular" bike? If the answer to those two questions is "no," definitely look into something with a flip-flop rear hub (fixed on one side, freewheel on the other) and brakes, so you're not dictating a Craigslist "For Sale" ad from your hospital bed the following week.