Hey there, Did Noadhibou to Choum, Atar, Chinguetti, Oudane and over to Guelb er Richat, north to El Beyyed Saumatre, then back along Sebkhet Chemcham, back to Atar over last Christmas and New year In a Defender 90 1995 300Tdi - its an excellent few days. You can do this without a guide, however I would not cross the railway tracks - keep south of them as there may still be mines around - it is not worth the risk. It was short of 500km or so over to Choum then rough piste to Atar - corrogations load up with what you need and head out - if you need to stop then there is Camping Bab Sahara in Atar which is ok but not that cheap, run by a Dutch guy.
Atar is generally quite expensive - especially since it gets lots of tourists through from the nearby airport and Nouakchott is only 6 hours away on the new road.
For waypoints, the book Mauritanie GPS proved to be an excellent source.
Obviously you would need 2 vehicles minimum for this trip, we met up with a German Land Cruiser and a Swiss Steyr 4x4 Truck in Nouadhibou who wanted to do the same trip - worked out fine. Yeah there are some stretches where its soft going, but in a well prepped 4x4 you should not have any problems.
There are no recognised fuel stops between Nouadhibou and Choum, take plenty, there are a few villages where you could blag some but be prepared.
Same for water on that leg, East of Atar there are more Bir's around and its easier to get water - keep your tank/cans as full as you can though.
Fuel is normally available in Atar, theres a few stations, be aware of the shortages and rationing that occur in Mauritania occasionally - It is not a good country to set your Itinery in stone - be flexible and enjoy it.
I got 24mpg on this leg or 8.5 km/Litre, we were making our own tracks on Gps as it was generally easier and faster than following the pistes
Oh Yeah, Keep an Eye out for Ben Amera - the second largest Monolith in the world over near Choum - its worth a look quite spectacular rising out of the desert.
Hope this is of help, Cheers Grif.