ET for 8.5J wheels is 45. I have the 18" 12-spoke wheels, they are not on 2006 brochure (only 19"). Not sure if they were available before or after 2006?
As it happens there is a refurbished set on ebay at the moment
http://www.ebay.co.uk/itm/Genuine-re...gAAOSw2GlXG~zO
My tyres are 255/45/18. Difference in the profile height is marginal compared to 235/50 so I wouldn't think there is much difference in comfort.