Some good advice here, rafmatt.
Mk4s, Alphas and HGUs seem to be the most prevalent around the 'scene' these days.
Survival Equipment Services at Tetbury (think they used to be across the road from Kemble when Deltajets were there) do everything from helmets to bangseats and ditching gear and have an excellent reputation.
I personally use a Mk.4B with a P Mask for both Tiger Moth and Harvard, to keep the external noise out.
I have also just had a Gentex HGU-55P rewired by Dave Lewis at Flackwell Electronics, near Aylesbury. This is a lightweight set up with a boom using an M7 Electret mike, compatible with UK radios. This is best for comfort and lightness and, with earplugs, keeps the noise of a Pitts or Extra down to sensible levels without the expense of ANR.
Dave did a professional job without charging the earth.
It seems though that, should expense not be an issue, the new Alphas are the bees' knees, especially with the warbird crowd, because of their lightness and up-to-date ANR electrics.
Custom paint jobs ? Magic ! In general, too expensive at the mo. A niche market if you can crack it with some spectacular (fireproof) designs ! Good luck.
