Have you thought about taking your bike *and* your 4x4?
It is such a long way to get to Mauritania, all tarmac, in my view it is best to get there as quickly and as comfortably as possible. Maybe you could put your friend's bike on too and share the fuel costs?
You could arrange to leave the car with a camping when you get to Nouadhibou, whilst you ride around.
Again, in my view, a bike is the best experience and fun in the desert. Plus travelling with few belongings makes for a better sense of harmony with the peoples and the desert.
I have always thought Honda were the most reliable of the Japanese motorcycles. If you can find an XL600R in good condition, it may be worth considering. My friend sold one recently (1991 model), we rode together on UK trails, surprisingly it was very similar in capability as my XR650R, but with greater economy (kpl) and carrying capacity.