Get a Snell 2020. Worth the investment now to be able to keep using it for longer than the 2015's. The 2020 models have just started entering the stores around me so they are available. I know you say it will be only once but like the rest of us track rat lunatics the first time is never the only time. You will be searching after for autocross, trackcross and HPDE events the minute you get home. Invest now and have fun for longer. Also, a proper fitting helmet is significantly more comfortable in the long run that one that doesnt fit properly because you get it cheap off the internet. Each have their own internal sizing, padding, strap and visor position so find one that fits your noggin the best. Dont be afraid to ask around for shops that will allow to you try on a few(i know the Rona has stopped this a bit).
Check OG Racing if in the DC/MD/NOVA area(they have an amazing showroom and customer service is the best ive ever experienced).
I bought my Bell M.8 helmet with 3 shields and they helped with fitting, cleaning it and installation of my visor of choice while i waited. Also assisted me in getting fitted for a new one piece race suit even though i told them i wasnt going to buy that day. They spent a good 3+ hours with my friend and I on a walk in without an appointment. When the time comes to replace my 2 piece suit ill definitely head back to OG Racing.