I'm supplying my own wheels to the factory because I don't like the current 'Vauxhall Corsa' wheels used.
You need to get an 18 by 8.5 with a 35 offset of 19 by 8.5 with 35 offset according to Gregor at the factory, however he was a bit uncertain about the offset without looking it up and he was busy with someone when I asked him.
The company that made the S2 and the 2006 S3 15 spoke wheels, Rimstock do a range of forged wheels that will fit the Aero or the Aeromax.
http://www.rimstock.co.uk/rimstockuk/fitmentmakesearch.phpIf you look up the Aero, they are showing a 30 offset.