I'd recommend a full face since they're dual purpose. Polycarbonate helmets offer good protection and are lightweight. Good composites take it up another level. Composites transfer more shock than poly's so they depend more on the styrene inner shell to minimize concussion/brain trauma. Composites offer better penetration resistance. Composite full face helmets will probably offer better chin/face protection as they should be less prone to collapse on impact with the steering wheel.
Good ventilation is important with any helmet, especially in a car in hot weather with no AC.
Generally there are two shapes of skulls, round and egg shaped. It's critical to try helmets on before purchasing. They should be relatively tight with no pressure points and left to right horizontal rotation (losness) on your head should be no more than 11 to 1 on a clock dial.
Brand names typically draw premium but there are some equally protective "off-brands" on the market, sometimes the shells are made by the same company that supplies the brand name manufacturers.....at least that's the case with motorcycle helmets.
Also consider face shield options available, tinted shield are better than sunglasses and offer good eye protection.
You want a liner and cheek pieces that's easy to remove for washing and most manufacturers offer multiple sizes of cheek piece to tailor the fitment.
The Bell Racing Sport helmet would be a good baseline to start from:
http://bellracing.com/helmets/pavement/autocross
The Simpson Bandit is another good helmet that's pretty light.
http://simpsonraceproducts.com/produ...id=10399&sort=
If you want to go big league Schuberth is a favorite in F1:
http://www.schuberth.com/en/