Recently (got back two days ago!) did a trip to Morocco and Iran on a 2003 XT600E. The bike was brilliant for the job, my first suspiscion that it was about as good as you can get for this type of touring was totally confirmed.
I thought long and hard over the fuel question but eventually decided to buy the 23 litre Acerbis tank (which actually holds a little more
) and have no regrets at all. With the limited space available on a bike, having the extra fuel in a tank rather than in bulky jerries was definately worth the £200. Even with the tank totally full the bike never felt unstable, in fact the bulk of the tank sits quite low over the frame.
23 litres proved more than enough and I was never in a position where I wondered if I would have enough fuel (except once which was my own stupid fault!).
I'd buy a centre stand, I had four punctures, and although it is possible to sort a puncture without a centre stand I can't imagine the arseing around in 35c this would have entailed would have improved my mood. SW Motech do my one for about £90, but it's actually a little unstable. Maybe someone else knows of a better one.
I'd fit a screen too, you can buy them apparently but I couldn't find one for the XT and made my own from some 5mm polycarbonate I bought from a plastics shop for about £10. Using a heat gun I put a slight fold down the centre of it to increase stiffness and bolted it to the headlamp surround using 3 M6 bolts. It worked really well.
As for luggage I used Touratech racks with EIBC aluminium boxes, a set up that proved totally adequate. It cost about £400 all up. I also had a hold all strapped to the back seat and this was a bit of a pain in the butt but did the job I suppose. In an ideal world I would maybe have had a top box instead.
For long distance XT touring the three things that I had trouble with were punctures, so carry tubes; a broken chain, my own fault for not checking its tension regularly enough and the front brake blocks needed replacing (not prematurely, just in line with service interval) so I'd carry spares.