Hi and welcome to the group...I also have Sulcata and I use Trex bulbs for most of my tortoises. I use 250 watts for my big Sulcata and 100 watts for my smaller tortoises...HTH
I think that a 5.0 should be fine.I do have one 10.0 over my box turtle and i really like the color it puts out.also the box turtle seems to be more active with the 10.0.Just make sure you only keep it on for 10 to 12 hours a day.