Bearings.
All of them!
If you buy it then regrease all the bearings ASAP.
Front sprocket wears at twice the rate of the rear - and is hidden behind a cover. Take a look.
Engine is pretty bullet proof, but early GS 650s can suffer from 'stalling'. New engine management chip sorts this out (raised tickover on the move).
Dakar is more likely to have been (ab)used on the dirt. So check the bearings.
All of them!