While Im all for the idea of resurrecting a sierra if the logistics can be worked out (it IS quite an interesting thing), an RS cosworth conversion is probably something that shouldnt be contemplated on this particular car. The main reason for this is that unlike say, the subaru impreza or mitsubishi evolution, their are very substantial differences between a stock sierra and the RS cosworth, probably the most important one being the fact that the RS was based on a specially strengthened and modified Two door Sierra.
http://en.wikipedia.org/wiki/Ford_Sierra_RS_Cosworth
as you can see,
That generation of Cosworth was exclusively two door so i doubt there will be a straightforward way to do a "conversion", short of maybe picking and mixing parts from the later generation which DID come as a four door....
IMO you should simply go ahead with with your restoration idea by finding a complete car and bringing it down in bits... If you can find one which has the V6 engine, you'll have a car that's plenty rapid enough for SL conditions, while still being interesting and different...
Anyway, best of luck mate, and let us know how it goes.