Re: 225/40-18 on a 147?
That recommendation might have something to do with German Tüv-regulations, 225/40/18 is a couple of millimeters larger in dia than the standard tire and that would affect the speedo and they seem to have zero tolerance for that in Germany. There should be no problems in using 225/40/18.
I also used to have 17"x8" rims with 35mm offset in my old 156 and there were no problems with them either, 18"x8" with the same offset should go fine, except if you have a GTA or Brembo calipers. Then they might not clear the calipers (depending on the rim design) as was the case with my 18"s.