Make sure you get the latest snell "vintage" helmet to ensure whoever you are tracking with accepts it for as long as possible....
Audi and BMW club look inside the helmets to check.. Seems like Snell 2015 will be out in October..
http://www.racinghelmetguide.com/aut...t-ratings.html
I have a Snell 2005, and that is still OK so figure 10 years life for most clubs.