Alpha Helmet
Just to note, the Alpha helmet isn't great if you want to do any long-ish sorties, like in excess of 1 hr 30 mins or so. Not so long ago I wore a Alpha helmet that did fit well, but after about 1:30 I got terrible headaches where the helmet stopped the good flow of blood over the top of my head.
If possible go for ex/current military type helmets, better fitting and they come with the added sun visor.
A.S.I