This is all you need for autocross (and yes, don't use a motorcycle helmet, different rating):
http://soloracer.com/pyrotectpsof.html
I use my Pyrotect for autocross/track days/TT and it is great and relatively inexpensive. I was hesitant to buy online without fitting one up, but it turned out to be no big deal, just have to measure your head properly. I had ridden sport bikes for years and always wore Arai because of my head shape, so I was really concerned about helmet sizing.