When you sign up for AdWords, you can decide to display your ads on content sites. It's literally a box that you uncheck, and then you are on google.com search only.

Purchasing AdWords and optimizing it is a science in and of itself. goodkeywords.com provides a useful tool for researching different potential key words. Then you can use G's sandbox or the Overture key word pricing to project ROI. You want to shoot for at least a 1% conversion rate on the AdWords advertising.

I have been out of this area for a couple months, but send me an e-mail if you have other general questions. Maybe I can help out.