Mk1 or Mk2 Focus?
2- Yes you can. Just need to buy some extra wireing. Ive used
www.caraudiodirect.co.uk on a few occasions and found them to be good.
4- Eibach springs are highly recomended for all Focus's. Will lower by 30mm. I had these on both my Mk1 Focus's with 15" & 17" alloys.
6- On a Mk1 Focus, 225/40/18 tyres are too wide with normal arches. Would need to use 215/35. On a Mk2 Focus, 225/40/18 is fine, same as the ST alloys. Not sure about the offset tho.
Andy