I did that road in a Mini in May 2013. Road conditions varied according to how recently each section was graded. Most of the time (between head gaskets!) we managed 80-90kph over the corrugations. It was very dusty and barren but not rocky.
There are roadhouses every 300-400km approx where you can refuel and even stay overnight - Warakuna, Warburton and Tjukarila from memory. I can look up the locations if you need, but there is information online.
Melbourne - Adelaide - Uluru - Great Central Road - Fremantle - Nullarbor - Adelaide - Melbourne in a $1000 Mini - May 2013. We did it!