15 offset would be nuts, you'd be better off with 35. Remember the lower the number the more the wheel sticks out. Stock wheels are et52 to 49. I run 18x7.5 et45 on my first gen. they sit nearly flush.
Focus MK1 RS wheels are 18x9 et42 they require fender work to fit on a 1st gen focus that's lowered.
Here is a set on a stock SVT suspension.
Here is a set on a slammed focus.
Here is that same wheel on a slammed Mk2.