arches are wider front and rear on focus rs so the bumpers are a little wider too. also the hubs are different and shocks and i think drive shafts are longer and certainly the pwer steering rack is different (and a LOT LOT LOT more expensive) so a complete rs conversion is expensive!

to keep costs down you could use the rs engine and box but standard 2l drive shafts (as the 2l has an mtx75 box) and use st170 front hubs with either focus rs callipers and custom brackets or a big brake kit for the st170. that way you wont need rs specific suspension kits. the rear brakes on the rs are the same as st170 rear or you could have the slightly smaller diameter (but same calliper) 2l rear disc brakes. this wold also mean you can keep standard arches and if you want rs bumpers then get some knock off ones that fit the non rs models.

thats all that comes to mind at the mo, sorry i cant help with regards to the respray.