For gearing, I'd say use what you have. If you end up not being able to source a replacement chain ring on tour you could just replace the crank set then with the money you saved by not doing it now. Every bike shop in the developed world should stock 8 speed cassettes. I'd just be sure to have a fresh one installed and possibly a spare in my luggage for crossing, say, central China. Worst case, you just might have to replace the entire drive train with whatever is available wherever you happened to land.

If you're still trying to improve braking, consider replacing your fork with something like the disk trucker fork and installing an Avid BB7 road disc brake, which is compatible with your current levers. As I'm sure you know, the front brake provides the majority of your stopping power; you could just leave the rear cantilever as is. With a really solid fork up front you could install a beefy front rack and shift your load forward, which will make the bike handle nicely and reduce that wobble you mentioned.

Lastly, while SON is the best money can buy; you can get a pre-built Shimano dynohub wheel for less than the SON hub alone.