Hi emyrose!
I'm sorry to be so late in seeing your post, but want to wish you the best on your upcoming trip. May it be safe, fun, thrilling, and full of adventures and memories to last you a lifetime -- with a safe return home at the end.
Looking at your schedule, I realize you're likely to depart very soon indeed, perhaps any day now, and I hope you found the bike you want and need.
Really, you can't go wrong with any of the Thorns in your survey, and I agree with Richard -- a Thorn Raven Tour like his would make a wonderful bike for your trip for all the reasons mentioned.
I have to cast my vote for the Thorn Sherpa because it is what I have and I am so pleased with it. The derailleur drivetrain would likely provide greater opportunities to find replacement parts along the way, but with the Rohloff-hubbed models, maintenance will be lower and they have such a stellar record for reliability, replacement parts would likely be a non-issue.
Please keep in touch with the Forum when you can to let us know what you chose and how you're doing. Can't wait to hear all about your trip "after" or even "during" if you have a website.
Richard, I'm right there with you, wanting to do the same trip one day; sounds fabulous and then some.
Best,
Dan.