As per the regs, "Helmets bearing “Snell Memorial Foundation” 1990 M or SA (or more recent) labels must be worn by all drivers and passengers."

However, especially if you are considering getting into other forms of racing, a Snell SA2005 is a good choice, since if you're in any vehicle with a roll cage, you can't use an M (edit: SA is also tested for some fun stuff, and is flame retardant)... Also, I'd say generally avoid used helmets. You can get some good gear, without dropping 800$ on a Peltor.

I'm sure someone would be happy to lend you a skid lid - usually there is a spare or two.