The above are excellent questions and suggestions, to start with. I'll add couple more:
What access to repair facilities will you have?
Are you going on a tour with a group or by yourself?
Camping or staying in hotels at night?

If I were to go on a tour without knowing a lot in advance, my default bike would be a mountain bike of some sort, as they are durable and versatile. But the more I know about the tour, I would pick more of a specialized bike and associated bags/accessories.