A couple of years ago Cleon produced a very helpful guide detailing what were the vital skills and other skills he looks for when signing a player in each position. I can't find the thread now, but I did photocopy it at the time. I have used it for teams at all levels ever since.
Unfortunately, due to an oversight he accidentally omitted what skills he looks for in a full back. So I would be very interested to hear from Cleon or anyone else what are the specific attributes (both vital and additional) you look for when you are looking to buy a full back.