I'm currently living in Kathmandu, Nepal, smack bang in the middle of Inida and Tibet.
It looks like the monsoon is going to start early this year, so depending on when in September you arrive it could well be over or at least majorly easing off. It was done with mid Sept last year and it came late last year.
Biggest problem with your gear in these parts is dust (end of monsoon is actually when the dust is at its best, but there is still heaps of dust!). I also shoot a d200 the body is great, more of a problem for non weather sealed zoom lenses, as they can suck the dust inside the lens - where you can't clean unless you or someone else pulls the lens apart. So be aware of it, avoid zooming quickly (as to reduce the vacuum your creating) and make sure you have a bulb blower, and other cleaning items that you prefer. Personally I use a bulb blower, brusher blower, micro fibre cloths and a lens pen.
Of course the most important thing in regards to your gear is travel insurance. Make sure (doube, triple check) it covers your gear to the nth degree for all possible circumstances.
Take the time to learn some of the local lingo (greetings, thank you's) and some of the cultural norms (i.e. they really don't like shorts in these parts), and when to use them. Not only does this show a certain level of respect but should find it will help you to get better photos, particularly of people.
India is somewhat ‘crazier’ than Tibet or Nepal, so be aware of your surrounds and use your common sense in regards to your security and the security of your gear – particularly so if you’re female (don’t want to sound sexist by any means but that’s the way it is).
As you no doubt would understand the lighter you can travel the more you will enjoy yourself. I would cull anything that isn't completely necessary and then some. If you find you overdo it you will be able to find most things pretty cheaply in India.
If you have any specific questions about region I would be more than happy to help answer your questions.
(Oh, and you should most definitely include sometime in Nepal on your trip! To be so close and to not to come would be most unfortunate!)