I'm afraid I have to disagree with Smaky on this one.
The Offset is the difference in mm's between the hub face and the wheel centerline.
(centerline = wheel width (not diameter) divided by two - thus giving you an imaginary line around the entire wheel which is dead center between front and rear of the wheel)
The bigger the Offset is then the farther the centerline will be away from the hub face meaning that the wheel as a whole may protrude the body work - which is illegal.
And, if you use spacers on the front then it'll definately protrude and just look terrible.
The 145 has a maximum offset of 38mm for this very reason. If you risk getting these wheels and they do happen to protrude the body work then the fuzz will stop you and give a "Notice of Rectification" which means that you have 14 days to take them off or face court action.
My advice is just don't bother and stick to the correct measurements.
As for the link, it definately works, do you have pop ups switched off? The direct address is: w w w alloy wheel guide. c o. u k
I've double checked my info with Wikipedia and I'm pretty sure i'm correct. If you're still not totatlly convinced then phone a store that supplies after market alloys, with a bit of luck they'll be pro's and be able to confirm everything for you.
No hard feelings to Smaky, but it's just so easy to get the wrong fitting wheels for you car which basically means wasted £££'s
(BTW. I've jst had 17's fitted to my 145 today
and they fit fine without any mods to the suspension or arches, don't think they'll take 18's although not 100% sure.)