You can snag a decent used touring bike for $400-$500 online via ebay/craigslist if you are patient. That's how I got my 1988 Trek 520, and it has definitely not disappointed me as a commuter. The nice thing about a bike that's actually built as a touring bike is that it has a stable geometry (that will help you to ride in a straight line, even at slow speeds), a long chainstay length (which gives plenty of heel clearance if you decide to stick panniers on the rear rack), and has front fork eyelets so that you can also install a front rack if you want to.

A decent sport-touring bike will make a solid commuter on the cheap (I commuted on my Trek 400 Elance for 3 years--I also toured on it, as you can barely see from my profile picture of it fully loaded at the Kansas State Line) but it really doesn't cost too much more to get a full touring bike, which is a much better machine IMHO FWIW. If you just attempt to stick a rear rack on your racing bike, you may find that it doesn't handle quite the way you want it to.

Also, fenders and mud guards on a commuter depend on your climate and comfort level. Here in South Texas where it is dry, they are not really needed. If you don't mind riding while wet and changing clothes at your destination, they are not needed. I used to have an old one speed with fenders, and I found that they got to be a pain in the rear when uninstalling/reinstalling the wheel (such as when you get a flat)...the tires always seemed to rub the fenders unless you had the wheel in just perfect. These were fenders that were organic to the bike, mind you, not a jerry-rigged installation that some chucklehead came up with. Anyhow, they got to be so much of a pain that I ripped the fenders off. I'm not saying all fenders are like that, but they were a hassle in my own (limited) experience.