My suggestion is to read all the links/pdfs at Adventure Cycling. http://www.adventurecycling.org/adve...-buyers-guide/

The only thing that really matters is that you have brazeons for a rack in the rear (front fork too, but only if you are really packing like a mule). The point of longer chain-stays is that you have heel clearance for the panniers. You can make just about any bicycle work. Only consider a dedicated touring bike as a necessity if you plan to go long distances without support while carrying a lot of weight. I have a Trek 520 and it's more than I need for a week long tour, to be honest.