You don't say what model @ your riding. Some of the ealier bikes (RD04 and earlier) suffered from premature wear in the wheel bearings and in some cases complete failure. This was due to the lack of a shield being fitted and the grease being washed out. If in doubt there easy enough
With regard to chain wear it depends on the type of chain and sprockets, type of lubrication used, terrain your driving through etc etc. I changed the standard chain and sprockets on my last @ at around 26,000mls, but I was using the bike mostly in the UK and Europe on well paved roads, I ride like an old man and had a Scotoiler fitted. In dry dusty conditions wear rate would increase significantly. Best thing to do is check the chain and sprockets before you return to the UK and if in doubt buy a new set.
I also like Trailwings. They give a decent enough grip in the wet and I get around 12,000mls from the rear. I've tried 'Enduro 4' and Siracs, both were good tyres, but didn't last as long.
Bon Chance Bob.