If you are going to be gone for years, you may as well buy the exact motorcycle you want new. Whatever you ride won't be worth anything when you are done, so what's a few grand in the scheme of things?
You should read Jupiter's Travels (both books). You'll learn a lot, such as:
1. You have to be able to pick it up out of the mud by yourself.
2. Hard panniers break legs.
3. Travel light.
4. Have a reliable parts source.
5. Plenty of cash accessable to buy your way out of jams.
I rode a 950A for seven years and some 60k miles. All over the continent and Great White North. I bought your little 250r last year and rode the CDR, Pony Express and scouted some of the GWT in AZ and UT. The 250 is equipped to travel the same places I rode the 950. I recently bought a KTM 690R because it is as light as my 250 and some 150lbs lighter than the twin. I equipped that bike for travelling as well, since it replaces the 950.
Here is what I learned:
1. Smaller is better
2. Comfort is important.
3. Reliability is important, but they all have had issues that delayed me
4. Roadside servicability is important and will be tested
5. Handling rock gardens and mud will define rider and bike's potential
6. Fuel range is very helpful. 200 miles range seems the minimum.
7. Know how to operate a trail oriented GPS and make tracks on laptop
I would not hesitate to take my 250 RTW. I would not take my old twin. The KTM 690R is great so far, but fairly unproven and parts may be an issue.
If I were doing it, I'd look pretty hard at the new Husqvarna 650 Terra. It is a more powerful and lighter version of the BMW single. It has Sachs suspension, a wide seat and a 58hp bulletproof fuel injected motor that has been proven for years.
Straight ahead and faster -Bo Weaver 1970
"There I was..." -Griffin Niner Three Hotel
"One day closer to a parade..." Jonny Gomes, spring training 2013