Bulala, if you already have 18s on your Zetec, you should already know the offset, but the offset should be written on there rim of the wheel somewhere, in the form of ET45, or 45ET, for a 45mm offset.
Stud pattern is 4x108mm
Offset is dependant upon the wheel width, and is somewhere around 50mm for a 7inch wide wheel I beleive.
The 42mm offset as said by many other people is just used cos it is a common one out there, and generally the largest one avaiable which is the closest to the actual offset required. For instance, the OZ RACING Crono Evolution wheels as seen on the WRC Focus is only available in 4x108mm 42mm offset, and people wanting to have these wheels have no choice but to use a 42mm offset.
There is no real ideal size you can run from 40 to 52mm.
I will personally recommend a wheel and Tyre
XHP Scortch 17x7 not 17x7.5 because it will void your
Insurance check with RTA if u want.Tyre wise either a
BFGOODRICH G-FORCE or MICHELIN PILOT PROCEDA
Awsum tyres for handeling wet wheather and responce
ring me at work (02)96864606 ask for James