My experience of riding an XTR in Africa leads me to agree largely with what Thorsten has said above. A couple of points:

There were 2 fully prepared BMW Dakar bikes on our trip. They both performed very well, but had cost around �11000 each to set up!

I haven't ridden the new Tenere, but it has the genes to do the job.
XTZ large tank; fine, but you can add 10 litres fuel capacity by strapping a �10 can on the back, or by buying petrol in bottles en route - then you can ditch the extra weight when you get to more urbanised countries.

I would probably buy an XTR with as much good quality protection kit on it as possible for about �2500 and spend the other �2000 on preparing it for the expedition. Making it comfortable for the long days and "drop-proof" in the dirt are two important issues. XTRs are easily bent back into shape if you crash.

How well do XTZs crash? What are they like to pick up when they have a full fuel tank?