The requirements arent that strict,
Your personal comfort level, safety preference, and budget pretty much dictate your safety gear.
I went to Full Throttle Racing in Nashville (dirt track shop) and tried on fire suits. I got a 2 piece single layer cheapy G-Force suit, and they hooked us up with some new but out-of-date neck braces. I wear regular cotton socks, leather hiking boots, cotton T shirt, and cotton boxers under it (stay away from non-fire resistant synthetics like Underarmor because it will melt to you in a fire even under the fire suit)
I wore a standared MX style helmet with goggles (tear offs are a MUST), on some of the really dusty sections it would have been nice to have a full face with pumpers and filtered air.
If you get a full face, go ahead and get one setup for an air pumper, because you will want one.
Also wore armored knuckle MX gloves. I caught some roost from time to time and was glad to have them. You don't want a rock breaking your finger during a race.
That said, of course a one piece double layer suit, full Nomex underwear, glove liners, socks, shoes, full face helmet, HANS device, etc etc... You can literally spend as much on safety as you want.
I'm realistic in that with my current setup, if I crashed and caught on fire, I'd probably die, but since my race truck had a 100% stock floor, fuel system, bed, gas tank, etc. I figured I was at less risk than driving on the highway in a stock truck...