I've just taken an R850GS 18months round South America, and before that a month or so bashing it up in Morocco. The 850 is rare but just the same as the 1100.

Didn't know much about the mechanics of it before i went. learnt the basics like servicing and picked up a lot on the way. But as said, very easy to work on with 6000mile intervels so you shouldn't need to spend too much time on it.

The few weak points that showed up on mine are well documented.

Broken rear subframe. (same on 1150)
Reinforce it before you go., and therefore before it distorts once you have broken it and ridden 100 miles with it off road! voice of experience there.

Final drive pivot bearings
Final drive, bevel box crown bearing and oil seal.

My pivot bearings seem to go every 20- 25K miles (carried spare)
My crown bearing went after 50,000 ( didn't carry spare as the manuals say it is a job only for BMW , but it turned out to be easy to replace them).

these are the same on the 1150 as well, but a lot of the crown bearing seem to fail earlier. As i think Margus says - think of them as you would a chain and sprockets and check and change them as neeed be, with the added advantage that spare ones take up less room and less weight than a chain and sprocket set.

With Conti TKC80's on they are ok off road. as long as you learn to pick it up.
However the indicator broke every time i dropped it, so changed them for bendy ones!

With the advantage ( in my opinion) over the F650GS that you can cruise round all day at 80mph on 500 mile days and still get 20 kpl/ 55mpg

cheers
mike