For posterity, I'm copying over my answer to your thread question on the other site:
I too would be interested in making my car into an Opel clone (which I guess it is anyway, but you get what I mean).
So, upper front grill assembly if it can be easily made to fit properly. Plus rear badging, wheel centers, maybe steering wheel cover.
Price would be the big factor, though. A company in Germany is currently marketing a 'kit' (they say will fit Saturn 3DR models) on eBay for $475 plus shipping.
http://cgi.ebay.com/ebaymotors/_Car-...1%7C240%3A1318They offer two other versions, but this particular kit's grill matches the Saturn's lower grill, having the same horizontal chromed bars as our cars. But frankly, I would not be willing to spend that much just to make the car look more like an Opel.