Eibach sells lowering springs that will lower you about 1.5" in the front and 1.7" in the back.
08 WP tC has a set of Canuck Springs for sale I believe, which are the same as the SSC springs I have, and that gave me about a 2" drop all around.
Headlights are the same for sedan or coupe, you can find them all over ebay and amazon. Whatever you like will fit. Though do some research on the site here, Some sellers/brands are far worse in quality then others it seems.
LED tails only come in one style available from eBay. I just put Spider Lites in mine.
I painted my stock rims and left em on for now. But I have the steels so it's a little different then yours.
As far as aftermarket parts they're everywhere. eBay, Amazon, just google whatever your looking for. They're are some dealers on the site here you can purchase from.
Check out G6Performance.com a lot of people and ideas there too. Also vendors who are G6 owners and have developed and sell parts for our cars