The tailgates are all interchangeable but best if you can find a good Mark one - some of the holes for the cables and washer pipes changed from Mk1 to Mk 2 and the metal around the window is thinner on the later tailgates, so your spolier will overlap onto the rubber.

Back panels should be available from Hadrian Panels or the like, just make sure you get a Mk1 panel because the light fitting holes are different, though I suppose a Mk2 could be made to fit if really pushed.