Probably better to go singlespeed over fixed, at least at first. You need a different sort of hub for fixed gears from what I've read, and a singlespeed can work on a regular hub. Surly has a fair bit of info on singlespeeds and fixed gear bikes (check the "Spew" section in the sidebar). They also sell the spacers that you'd need, so any bike shop should be able to order them.