Hi mate, I am not far from you, I have a pug 205 with 4x4 cossie under it, the car was converted by Djm so feel free to come and look to see how it was done. There is a converted puma on here too and that has cos 4x4 too, local to us as well.
Coley on here has a good thread on his focus build and that will show how to solve a lot of problems
I can put mine up on the ramps to show you.
It can be done and needn't be that hard to do, the main bits you will need are the gearbox tunnel from the 4x4 and the rear beam etc (you may want to consider a 6 degree beam from the start ?- depends on budget) You will need the front cross member from the 4x4 and then fabricate in the bottom of the chassis to locate the cross member.
Prop shafts can be bespoke from the place I got mine done near Heathrow -prop shaft services.
You will need to decide on bladed anti roll bar OR just use the 4x4 anti roll bar (I have one that you can have) the 4x4 one helps locate the track control arms on the front.