the Hellfire SS was discontinued months ago so you will probably find it on the discontinued cars area.
I believe some differences from the RTR and the PRO are:
lighter chassis
aluminum chassis braces
wing mount
race tires
cnc engine mount
and I can't remember others, I would watch on the diffs, I had to sell my Hellfire because there was a point where I could not finish a race in my track because of blowing the diffs.