The only thing i would argue on that is the bit about the doors, i'm 99.9% sure they are the same. For instance cabriolet doors, Ford sell the same door for MK3 & MK4, same with S1's and S2's.

But as said, its a lot of work, and changing a MK4 to a MK3 is a right slag of a job, and any value it had would be forfeited anyway. Find a tidy MK3 shell and convert it, thats the best way IMO.