Where to start?
Well I think the F650 is a great bike! I've just come back from a 9000 mile trip to Gambia and back (see thread on Sahara Forum) and my bike was perfect! Didn't even get a flat (but I'm not sure that is down to the bike!
Took in lots of tarmac (longest day was 680 miles), some really rocky/sandy pistes in Morocco and three days of riding in the Mauritanian desert. With a nice deep flood in Morocco to wade through just for good measure. The F650 took it in it's stride, a great all rounder.
There is a wealth of info out there, some of which you may be aware of, but a great starting point is http://f650.com/
dedicated to, funnily enough, the F650! The FAQs have loads of info on mods etc and very helpful maintenance info! A must for every F650 owner.
Another site is http://www.smellybiker.com
He is travelling/has travelled with an F650 and gives the low down on mods he made; those that worked and those which didn't.
For me the following worked:
1. Touratech tank. As bmwbec said, it is expensive, but I wouldn't be without mine, especially in Africa.
2. Handlebar risers. If you are planning to off road then these are a must, unless you have arms like an orangutan!
3. Touratech (TT) Chainguard. Ditch the plastic cr*p together with the "possum scraper" for the reasons bmwbec mentioned.
4. Subframe seems to be really strong as standard. I had my 35L TT panniers and a roll bag on for my trip across some really rough terrain and it held up OK. However if you are planning on taking as much kit as McGregor and Boorman you had better look at an RSJ!
5. Footpegs/gearshifter/rear brake. I managed to get the TT Rallye pegs/shifters/brake from a guy on ebay. A good piece of kit. It all depends on how much off road/track you intend to do. IMO the standard footpegs are not up to it, they are very narrow and would prove uncomfortable for long periods of standing. Having done the BMW off road course in Wales I can say that the standard brake lever will end up shaped like a banana if you fall on it and the same goes for the gearshifter. The alternative, cheaper option, of course is not to fall off!
6. Suspension. Standard set up on the Dakar worked for me. Even with full TT tanks (39litres) and equipment the forks never botomed out. However if you are built like Robby Coltrane I cannot vouch for the reliability of this information!
Well, that should get you started!
Oh yes, Continental TKC 80s work as a good all round tyre(if you need off road capability as well) and gave me about 7000 miles on the rear.
Any other questions give me a shout!