125/150/200: which one should I get?

I'm planning to buy my first bike, for the purpose of exploring China (mostly rural areas), and camping. I'm not a speed freak, but I would like to be able to ride uphill at a reasonable pace. My bodyweight is 64 kg (141 pounds), and I will pack light. Of course I want a bike that can endure long-haul trips. My ideal price range is within 8000 or so.

My feeling so far is that a 125 might be too weak uphill, but since it's my first bike, 150 should be enough. Thoughts?

Also, is engine displacement even the best indicator of performance, or rather, should I be looking at max power (8/10/12 kw etc.)? And, why is that some 150 bikes have a max power of 8 kw, whereas others have 10? That seems like a rather significant difference for two bikes with the same engine size.

I would always go for the 200's, not for the power which tends to be only a little more than the others, but for the low-down torque or pulling power. You'll also find that the weight is more or less the same as the smaller engined versions, as they use the same chassis.
The engine will less stressed and should last longer.