You simply cannot go wrong with the Bell M4 to start off. Once you get serious, then upgrade to something lighter that is carbon. The Bell M4 has very good ventilation and great visibility (about 0.1lb heavier than the Simpson Super Bandit). You can't beat the sale price from Safe Racer.
On size, it's hard to find an auto racing shop let alone one that carries all brand helmets. The rule of thumb is auto racing helmet will fit 1/2 size tighter compared to motorcycle helmets. So if you're a loose medium motorcycle helmet wearer, medium in auto is perfect. If you're a tight medium, then go to a large. Safe racer will exchange the size for you as long as you didn't go out on a weekend with the helmet. Just remember the padding will break down, so the first fitting will be very snug.
Safe Racer has great customer service.
http://www.saferacer.com/bell-m4-sa2...-racing-helmet