I agree with buying racks locally-- you get to mount them and see if they work. Sometimes you need a special rack for a bike-- some rear racks have set backs to stop heel strike, front racks mount a couple of different ways, but most bikes can use the simple cheap ones. I'd look for a racks that mounts from the brake bosses on front and from the seat stays in the back.
Tubus and Surly make very good racks that cost $$$$. Buy them if you're about to ride around the world. If your have less agressive touring plans, buy cheaper racks and panniers. They'll work just fine if you're not totally flush with cash.