Guppy's advice is pretty comprehensive and, had I the cash at the time, I'd have gone for the options he mentions on the -55, especially the ear cups. The one downfall of the -55 is that it has very little noise attenuation, to the point where I fly with earplugs now.
I would only add a caution on the idea of buying a second hand helmet. You never know what's happened to the helmet and if it's been dropped it may be useless, but you'll never know that just by looking at the surface.