Check out Hexcrews.com, they make grade 12.9 screws with a zinc-nickel coating that's corrosive resistant. I haven't found a better option than these in terms of strength, cost and appearance. They also eliminate the button heads with cap heads which allows you to use a larger wrench size. If you car isn't on the screw kit list it can be created in a few days. Just make sure you have a quality set of wrenches because soft tips won't last long with these.
http://www.hexcrews.com/hexheads-screw-kits.aspx
http://www.hexcrews.com/hexheadsap-factsheet.htm