Alright mate..welcome to TorqueCars! I agree with Steve, if you don't need the extra space then buy a salloon. However, if you insist on keeping the Zafira then how about having the exterior de-badged and smoothed over. A re-spray could also do wonders if you pick the right colour. Another touch you can add are a decent set of rims. As for interior...if you have young children how about some monitors in the head rests and a dvd player? Could keep them quiet on a long journey!
Maybe have the seats re-covered as well and the dash colour coded. Right, performance, you won't be able to turn it into a super car. Your first problem lies with the weight...heavy car = poor acceleration. You may be able to fit parts intended for the Astra or Vectra though...all depends on how much power you want?