I never take much because quite frankly, you can get 95% of what you need out on the road.
This is my list:
FPC
EWS ring
Oil filter and tool (it's good to keep the oil clean especially if it's hot and dusty)
Good tool kit
Puncture repair kit
Duct tape
Cable ties
Haynes manual (for reference by any mechanic if it goes horribly wrong, which it won't)
Clutch cable (cable tied to the original so it's simple to change in an emergency)
Good first aid kit
As already said, make sure the bike is fully serviced and things like brake pads are in good condition.
There are plenty of BMW shops in the states where you can everything you need and a good service when you get there. They don't tend to rip you off either
I wouldn't bother with anything else TBH as in my experience, the 1200 will take a fearful hammering and will get you where you want to go