Keep in mind that the majority of event planners will insist that you wear a full-face helmet if you plan on using an aeroscreen.
If you're going to acquire a bike helmet, you may as well get an MSA cheap one. It's roughly the same price, but, as Nick points out, it's made specifically for automobiles, so it's one less thing to worry about financially if you decide to dip your toe into the Speed Series.
You'll find the perfect match with Luke at Playskool Motorsport.