The only scooby bit you appear to be using is the rear diff?
I really fail to see how that makes the project any easier than using a cossie one you can buy for a couple of hundred quid anyway.
you seem to have TOTALLY forgotten about the steering?
Have you seen how different the mk5 shell is to a cossie shell, which is basically a sierra.
If you want to do it on the cheap and dont mind fabrication work then do the following:
Buy a ford sierra
cut the shell off the floor pan
Buy a ford escort
cut the shell off the floor pan
attach escort shell to sierra floor pan
then all the cossie gear will bolt straight in
if you want to then fit an imprezza rear diff, feel free.