First off, do you understand the difference between a FG (fixed gear can't coast) and a SS (single speed can coast)? To my way of thinking, you'd be better off with a SS and find it easier to do the conversion with your old road bike. I'm assuming from your description of the commute that you don't need to change gears much due to the flat terrain, but do you have to face strong headwinds? Where I live, it is often very wind to the point that riding a bike with one gear for long distances is not practical.