Helmets are like race seats, better to try them on for fit before buying if you can. Try local race shops or friends. Helmet designs will fit differently and will size differently depending on your head shape (round, oval, long). Price and design options range from low-medium end (e.g., Pyrotect, Racequip, G-Force, HJC, Impact Racing, Simpson, Bell, etc) to mid-upper end (e.g., Arai, Stilo), and can depend on weight, features etc. You'll get different opinions on open versus closed face, with some folks pushing for open especially if you still have airbags in your car. I started out in autox with an open faced but have always used a closed face helmet for track days and racing. You'd be surprised what kinds of crap can kick into your car/face in a track event (especially bits of tire rubber), plus it helps keep you warm/dry on cold and rainy days if you run with the visor down.
