i think its ambitious
if i wanted a 4x4 7 i would use the gearbox and front diff and hubs off a 4x4 sierra. you would then have to move the front dampers forward so that the drive shafts would clear. something like the mk indy R front suspension design would work.
personally i think is a bit of a waste of time using the zetec as it doesn't make enough power to warrant 4wd.
i dont really think 4x4 fits the ethos of a roadster is there a particular reason you want 4x4 or is it just for the challenge of doing it? i can tell you before you start that a standard roadster will run rings round it all day but i doubt that will put you off.
what im not going to say is dont do it. if you think you can pull it off then i personally would like to see.