I think that the market for the actual SPAS 12 would be more lucrative, everyone I know is fed up with lightweight plastic ones. there is a lot of interest in a more realistic weapon, I have seen many SPAS 12's on skirmishes but no once come across anyone with a Pulse rifle, so it a shame that you are going to manufacture for these first.
I have bought the foldign stock and hook from Ocatagon and it really sets it off well, just need the metal handgurd and happy days, metal reciever a bonus but not as critical