I would invest in a body with some weatherproofing, or at the least a better build than the D40 such as a D200.
I have the Nikkor 18-200 VR, and it is a great lens im finding, I only take it off of my camera for macro shots, where I use my sigma 105 f/2.8
If I were going on a trek, I would want a weatherproofed body, or one with a good build quality, along with a 18-200 to cover wide-angle to telephoto. If you have alot of disposable income, which I do not, then I would suggest buying a D200, and a Nikon 18-200 VR. If you want to save some cash, or just don't want a $1500 of photography equipment to get damaged, then I recommend getting a third party version of the 18-200, just so that if something were to happen to it, it wouldn't be a $600 loss.
There is my two cents.
EDIT: And if you are traveling around the world, you will see alot of varied wild-life, and the 200mm reach of the 18-200 would be extremely helpful.