I wouldn't buy a rack mail order. Fit the thing before you buy it. Buy your panniers at the same time, load them up, test ride them for heel clip.

Most of the time your LBS will really help you with getting the right stuff. It's going to cost your more, but getting the right stuff is worth it in the long run. Don't try to go super cheap on stuff like racks and lights. I'd rather ride a yard sale bike with a top end headlight, fenders and rack than a new bike outfitted with crap.

The weight ratings on racks are totally worthless. Stay away from seatpost racks, get the old school kind that mounts from the frame. A good Blackburn or Jandd rack will hold a person sitting on it. I don't suggest this, but I've seen it done. I like the new Topeak stuff also, the rack/U-lock/trunk/tail light/pannier matching sets are really cool. Hook this up with SKS fenders and a Road Rat headlight and you've got a great commuter bike.

The only bad news is that all that good commuter gear listed above could cost as much as a cheaper new bike. Throw in some top end raingear and it's medium priced bike. But for 356 days a year commuting, it's the gear that really keeps you going.