Welcome!
The fact that you want to travel the world on a bike but expect it never to break down worries me. May i sugest you choose your bike now and start preparing it for travel. You see, it doesn't mater if you get the BMW, Kawasaki, Honda or whatever, they ALL BREAK! They all need their short comings remedied and you need to create an intimate relationship with the machine which will become your most important possession in the entire world.
I work as a BMW mechanic part time and have built a few travel bikes. Having owned a 1200 GS, i would not recomend this bike for world travel, for reasons that were stated earlier. On the other hand, an older airhead, like the R80G/S or even the R100 GS, with the proper preperation has proven to be an excellent choice, along with the KLR's, DR's, XR's and XT's, but remember, ALL these bikes need to be studied, scrutinized and prepared for travel before you can proceed. There is only one simple answer to your question, buy a bike that makes you comfortable and puts a smile on your face, research it's shortcomings and prepare it for travel. Make sure you build a proper toolkit and try to get informational CD's on your model. Prepare your luggage and make sure the sub-frame is up to the task, otherwise re enforce it. Low horse power air cooled motors are usually an excellent choice. No bike has a real advantage for parts availability so consider using cleanable air and oil filters. Try to stick to the more common tire sizes, like 21" front and 17"-18" rear, the oil head BMW's have a big disadvantage in this area. Never under estimate the importance of a good skid plate! Make sure your charging system is ready for all the accesories you plan on bolting on, like lights, GPS, heated grips (i recommend them) and heated jacket. Trust me on the heated jacket, you won't regret it
If you buy a chain drive, carry lots of master links and only use a good chain like DID. Toss the alloy sprockets and replace them with steel or stainless steel if you can. Learn how to lubricate it properly, there's lots of opinion there! If you choose a shaft drive, make sure you get a greasable drive shaft and learn how to maintain it. I have around 100k km on mine now,. but i love to work on my bike
Prepare your suspension! Most bikes come with junk non-rebuildable shocks, except for KTM's WP. Ohlins has a good reputation, along with Wilburs, which is my favorite. Wrap your shock spring with a Shock sock to keep out dirt, this is the #1 killer of seals. I like to also cut a piece or rubber from a tire tube and drape it over the shock using a hose clamp to attach it. Protect your front forks with Gaitors to protect them from rocks and such. Consider your bike will be heavily loaded with stuff, adjust or replace your forksprings accordingly.
As reliable as the Japanese bikes are, their handle bars and brake lines are for the most part junk. Pro taper or Magura and stainless lines are a good investment, along with aluminum insert hand guards to protect your fingers and handles.
Do some research on your brake pads and rotors, they will cross reference with other makes of bikes making it easier to get them in foreign countries. I met a guy who thought he could only get brakes for his BMW from the dealer! Spoke wheels are superior to cast and usually repairable. I love the tubeless spoke rims from Behr of Akront used on the BMW GS's, very strong and reliable along with the ease of tubeless repair using a plug. Regardless, you must carry a tube in the event if a side wall puncture which is sometimes unrepairable. Purchase an air-pump, i like the Slime pump. Cheap and good! Practice repairing your tires, you'll need it down the road!
Finally, don't read too much into the bike bashing that goes on here, it's a waste of energy and really doesn't help you. One bike is as bad as another if neither are properly prepared for travel.
Enjoy!
BTW, i really like that new 660 Tenere, it just might be my next one. I'm looking forward to some good reviews in the future.