Hi John, Light is right of course but you need to be able to keep the bike going - that's the problem isn't it. So far we have found it harder than expected to get parts in the Americas. The XT isn't sold in the USA so no parts are available unless cross referenced to other bikes. You really do need the workshop manual and parts list down loaded from this site. We had some success googling part numbers - got swing arm bearings from a Catapiler dealer!! I expect you have a service plan in mind but I would take one more oil filter than you expect to use and a few oil plug washers. A gasket set could save the day and takes up no room. The XT is sold in Mexico in theory but none are actually sold so we couldn't even get parts in Mexico city!
The XT cush drive rubbers are crap, you will find that new ones will go slack after a week, Take a piece of car inner tube to do Kev's cush drive mod on this site - it works a treat. Sand also kills the cush drive so a spare could be an idea, we both needed replacements after 6000 miles off road.
We rode the trans west Virginia trail (1200 miles of gravel and dirt following the Appalachian mountains) then the Trans America trail 4800 miles of gravel/sand/dirt/mud and rocks - we used one set of tyres that got us to Colorado springs then another set got us to the west coast and about 1000 miles of road. We changed each set before they were slick, probably say they were road legal but offered little grip in sand etc. I'm guessing you will be picking up tyres in Istanbul - have a look at Michelin Sahara 3 and Hiedenau K60 scout's.(we fitted Heidenau's in Baja and so far have been good - look half worn on 4000 miles) Both good for mixed roads of sand and gravel but not mud. To be honest TKC's are poor in mud, DOT nobbly's are far better and T63 rears run at about $65usd and out last any other DOT tyre. If you go for Heidenau K60, go for the 140 rear - it's a heavy duty tyre.
When you fit the rear shock, it's a pig as the top bolt is in a recess in the air box - you need to remove the rear fairing and loosen the under tray - you then only need to disconnect the air box and push it back. You don't need to remove it.
I was really pissed with the K&N filters, they are ok for road bikes and maybe dirt bikes in muddy Europe but they let fine dust through to the clean side - enough to leave a visible finger mark inside the air box. I'm a bit anal about service and cleaned and oiled the filters regularly. Take your K&N out and hold it up to the light, bet you can see pin holes through it. Fine sandy dust will not be good for the valves, The Pipercross filters were the cheapest foam filter I could find and don't let dust through.
Sprockets for the road we used the standard 15/45 but off road we used 14/46. The 14/46 combo is great for technical mountain passes etc but a pain on the long flat bits and on the road a right drag, 55 mph being cruising speed. We ended up changing back to the 15 front with the 46 rear for off road and it's a nice compromise. It's also easy to just change rear sprockets and leave the front alone. If you buy from B&C express get the renthal steel fronts, they last for ages. I know you don't want to add weight but a main stand is worth it. Our head bearings have worked loose a couple of times so worth keeping an eye on.
Just thought, while you have the air box apart, look at the low position of where the crank case breather enters the air box. You might want to plug the box and re-route the breather up under the tank with a small K&N fitted on top. Amazing what you can do with a plumbers push fit cap and two 15mm copper elbow's.
All in all the XT's have been brilliant bikes, tough,frugal,easy to work on and even loaded have taken us over rough dirt mountain passes. Angie has fallen many times and the bike has taken the knocks with ease. You picked the right bike. PM me your email and I will send you some stuff you might find helpfull.

Any way I can help, just ask - pleased to be paying it forward.

Paul