Graham Forsdyke is considered an expert on FW's. His site is the first one listed by oregongirl. He also moderates the FW site on Yahoo and answers a lot of questions there.
Preparing yourself is a good idea before buying, but you can still be looking for a great deal and snatch it up even before you know much. One tip-- if you are trying out a machine at a garage sale or wherever, the machine threads from right to left.
Enjoy the hunt!