I was thinking on getting some 18 inch Focus ST wheels for my estate to modify it a little (no other reason) but I was wondering will it fit under the arch as I know it will fit on the hub as its the correct stud pattern. If it helps I am currently running on Zetec alloys fitted by a PO.
Yes they will fit no problem, you could even lower the car afterwards using ford approved 30mm lowering springs will bring the ride height around 15mm lower than it currently sits before fitting the 18's
However your acceleration on 18's will be slower due to the additional unsprung mass
Bigger wheels on the hubs require more power to turn,
So your 0-60 will be slower
However handling, grip and stability are all vastly improved.