It isn't a bad idea to use the MTB. For one thing they're slower than a road bike and so less tempting to ride too fast on your tour. Secondly, they have a lot more trail in the fork and so the handlebar bag and low riders will have less effect on the steering under way.

They also have longer chainstays so that there's more heel clearance than in a cyclocross conversion.

True touring bikes have to be obtained from special sources now like custom builders such as Bruce Gordon or Mercian or Bob Jackson or some REALLY nice makers in France (I think that Alex Singer company is still in business and there was a semi-custom maker in Paris near the Venice Gate that had some beautiful Randonneur specific models). Although they aren't nearly as expensive as you would expect if you're handy with your tools. For instance, a Mercian touring frame and fork start at 375 pounds which is something like $700 for one of the nicest touring frames in the business. http://www.merciancycles.com/frame_king_mercia.asp