Well, to start, don't discount your MTB for touring, unless it's like a downhill rig or FS. I use a hardtail MTB (an old '95 Marin Team (Tange Cromoly steel) for commuting and long distance touring. Kind of like the SUV of tourers since a lot of time, I am going down dirt or gravel roads so it's kind of nice every now and then. Plus with slicks, it's got a decent clip if your in shape.
Anyway, rather than rehash the wheel:http://www.adventurecycling.org/features/howto.cfm
Scroll down and look at how to buy a touring bike for the road, etc. etc.
Certainly look for bikes with eyelets for racks and fenders for the front and rear. Look for long enough chainstays for heel clearing the rack. A longer wheelbase may make for less abrupt handling and check the rake of the fork for again, more predictable steering. MTBing usually requires a nimble steering bike, touring is not always advantageous.
pricewise, anybody from Trek, Giant, Surly, Rivendell, Co-motion, Bruce Gordon, Specialized, Fuji. name your price range and all sorts of models can appear!