Note internal hubs are not necessarily the ideal for touring, especially when off the beaten track. If they fail, they tend to do so in such a way that can leave you unable to ride your bike. You can't change gear while pedalling, which can be a pain on hills, and they tend not to have the greatest of range between lowest and highest gear, unless you fork out a lot of money for a rholoff. Personally I would go with a derailleur system as you can cobble that back together, even if you have to make it effectively single speed because of a broken derailleur, or mangled chain or some such. You may want to consider one of the modern 1x (pronounced "one by"), which have a single chain ring at the front, and 10-11 gears at the back. This has some of the same disadvantages as a hub gear in terms of gear range, but it has the ruggedness that a derailleur system offers.
Why 26" wheels? For years the common advice has been that if you're going really off the beaten track (think south america, or Mongolia) then 26" is the way to go as the spares are likely to be easier to find. But in the developed world, 700c is just as well served really these days and is likely to come with better rolling resistance, and ride over bumps better.
Beyond that, take a look at things like the Surly troll or the Genesis Vagabond, both are available as fully kitted out bikes, or as frames you can build in your own image.
No. Many mountain bikes, especially full suspension ones, and many road bikes won't have the fittings for a rack, so you can't fit panniers. That said these days with modern bike packing luggage you don't really need a rack. You can get a 20l bar bag, a 13l saddle bag, and about 7l of frame bag, and have your 40l of stuff, all while keeping within the plane of the bike (at a cost of raising the centre of gravity).
Hopefully that gives you some ideas to work with
J
