Have a look at our bikes on our web site, looks like we had the same ideas on a lot of things.
We are 16000 miles into our trip, going the opposite way round to you so we could cross paths if we keep in touch. Shipping to NZ now.
Take all the service spares you can carry - if you can't get Yam spares on route try cross referencing Yam and KTM part numbers for brakes - The Raptor 660 quad shares the same engine so in the Americas you can get oil filters and such.
We did about 7000 miles of dirt and found Michelin T63's to be very good and lasted well. A T63 rear with a MT21 front very hard to beat on mixed gravel/dirt/paved roads.
The RK chain tool works and lasts - take spare front sprocket tab washers.
You can carry a spare sprocket behind the numberplate, zip tied to the rack cross bar.
We gave up using fully synth oil and no use semi synth as it's much cheaper and easier to get, the bikes also feel better for it and burn less of it.
Before you leave take out the swing arm, it's easy - the inside seals pop out and the bearing goes dry. It's a crap design but if you re- grease it and keep in eye on it etc. We have a spare set of bearings and bushings now.
A set of Aerostitch tank panniers with a gallon can in each is cheaper than a big tank and balances the bike nicely, you can also fit wet gear on top of the cans.
WD 40 is great for lube to get tyres on and off if stuck.
Don't use K&N filters, they let dust through - Piper cross do a foam filter that works. We left with K&N's but they were crap and have had no probs with the Pipercross ones.
The tall screen is the way to go, you won't be going over 65mph much, fully loaded and thinking about fuel consumption. You can get 230 miles out of a tank @ 65mph, it drops off loads any faster than that. If you got into a death weave you need more weight on the front (tank panniers). Changing the sprocket gearing for off road loaded is worth doing, the rear sprocket at least, you will struggle with road gearing off road in the dirt.
I could go one for ages but might be telling you how to suck eggs, sorry if I am.
Have a look at our site and if I can help drop me a line.
www.twobikesrunning.co.uk
paul