Flying Anorak - to answer your question rather than subject you to a diatribe of whether it is right or wrong to wear a helmet in a GA aeroplane:
I had 2 x Mk4 when I was flying the chippie (1 for my passenger also).
I brought them on Ebay and then had a friend of mine (an Ex-Squipper) overhaul them. He changed the avionics in them as the Mic was not compatible with the GA Avionics in my chippie.
I think I spent £200 for each on Ebay, a drink for my friend and £70 for a microphone per helmet.
I found them comfortable and easy to use in the Chippie, I felt safer with one on and also warmer on a cold day !!, I rarely used the visors although mine were both operational and had soft covers.
When I finished I sold them on Ebay again for £475 each.
There is definitely a Market out there.
Don't be put off by what people say about image and poseurs, If I was able to wear one in what I am currently flying I would certainly do it. It's a bit like wearing flying gloves - some people say I am a poseur for doing that, for me they are just another piece of safety equipment........
Regards
Arc