I would recommend to use MahApps.Metro to achieve metro look and feel almost instantly. Have tried it, awesomely simple.
UPDATE :
MahApps.Metro licensed under Microsoft Public License - MS-PL which allow commercial use according to this SO question.
You also have some other options available such as Modern UI For WPF and Elysium. But I still recommend MahApps because it doesn't change much of the way you code. You can use standard WPF control and MahApps will style those controls to appear in Metro look-and-feel.