I would strongly suggest that you take a 4x4 into Mali, in October, after the rainy season many of the roads are no longer roads just deep and solid ruts. Height clearance of the chassis and diffs are important, in standard 2 wd, you will get wedged in. As for the dried river beds and vertical hill climbs, a 4x4 a must. We took the very bad old road from Kayes to Bamako in a 1982 Series 3 Land Rover and managed but only just, with some very tense moments.
I have been told that there are many new roads but for me Africa is the beaten track and only a 4x4 will do.