There are racks that mount to a seat post or use p-clips to attach to the seat stays, but they're not made to carry a very heavy load. Since you're shopping for a new bike now, I'd get one that's properly equipped. You've also got to consider clearance; if you're planning to use panniers then the short chainstays will virtually guarantee you end up kicking the bags while spinning.

If you're looking to go with a Specialized bike, consider the Langster instead. The chainstays are a bit longer and the frame has mounting holes for a rack. You probably won't be able to go much bigger than 25mm tires if you want fenders, but you will have the option. There are a lot of great single-speed commuters out there, but if you don't have much experience as a bike owner then it's probably best that you stick with your LBS' recommendations.