I also build super guns, but more in a cost effective kind of way,ie not with lots of unecessary features. I have owned so many different superguns and can sfley say that raven and arcade heaven are almost identical.
What ever you do, do not pay the outrageous price of almost 200 quid, cos thats just to much for a supergun as basic as that these days. They dont even have the scart connected to the speaker on the newer version, although the rgb adjusters are a nice touch. They claim some boards can damge the telly speaker, so they have installed one internally and the out put is abysmal.
I have had the datel supergun since 95, and never in all this time has my telly(S) experienced any probs.
Also, 70 pounds for a converted dreamcast stick is way to much. As an offer, I would do it for 15 pounds if you provide the stick. Remember, there is nothing major in a supergun, mostly apower pack and wires, and perhaps a cooling fan if they can be bothered.
The best way to go about it is learn yourself, try google and you will find all your jamma inquiries catered for. You may even be able to perform your own modifications after a while. For example, the D button isnt wired on the first raven super gun, and the credit button is located on the machine, not the joystick. I bet most of the super gun owners out there have taken care of small details like this.
Anyway, I love to listen when people are talking about stuff that im ignorant of (which becomes more and more everyday for some reason- HOW!?), and I also love to give advice or help about the things I do know about, so let me know if I can do or say anything to help you man.
Superguns=hardcore