Just to add to this topic now I have an RS kit.
My initial post sums up the type of focus needed, in addition to what I listed you will also need
2008 onwards bootlid - Not totally necessary as the earlier ones will fit, but the profile of the back along with the number plate area is different.
2008 onwards mirrors. Again not totally necessary, but they will finish the look. 2008 on have indicators in the mirrors and kerb lights, so these will need wiring mods to make them work properly.
The front grille will need a modified lock mech. The standard ones are built into the grille moulding which wont fit into the RS opening. This should be relatively straightforward or if not I was thinking of using a remote release.
Rear lights will fit from the donor car, but are slightly different than those used on the RS. Im leaving mine till I find a cheap set on ebay, but they are just 2 screws and 30 secs to change so easy to do later.
Rear fogs are different so will need post 2008 ones. You can get a uk and EU reverse light and use a red bulb in one so that both are clear and look better.
Arch liners from originla car look like they can be used, but may depend on your wheel choice.
Rear spacers arent needed but a larger spacer block in-between the hub and axle pushes the rears out. These are available form ford or ebay.
You may need to remove the rear side windows which means working near the rear airbags. I am fitting the side panels without removing the windows so hopefully wont need to touch the airbag area.
I havent done much other than dry fit the panels as I have other cars that are higher priority, but so far everything fits together perfectly. The front end slots together and on the car in a matter of an hour or so. Captive nuts are in the rear spoiler which is fully assembled and these are perfectly positioned so it takes only a few mins to swap.
Will add some more pics soon, but any time I get time to go out with the camera its raining!! Car is outside at the moment till I restart work on it.