Sudan is really a great country for biking, nice people and beautiful landscape, but the bureaucracy can get heavy.. just smile nod. As for your fuel I would say you need to carry at least another 5L with you. I can t remember what the furthest distance between refills was, but there were times when i I came close to running out on a 28L tank. Remember also that sometimes you will get to a village with a fuel pump, but it will be empty which can really mess with your schedule!
As for sand; you can do the trip easily without ever hitting a lot of sand if you choose to stick to the main road the whole way (Nile route), there are plenty of opportunities to find sand if you want to. Some of the "shortcuts" are very difficult but also very rewarding.
Hope you have as much fun as I did!
"If we don't find something good at least we will find something new." Voltaire