Duplicate Content means like you put the same phrase on your page multiple times trying to get it as a keyword hit. You would be spamming if you put too many keywords per page. This is keyword spamming. Duplicate over multiple pages is fine. Think like a company tagline on everypage, it isn't considered spamming. Really duplicate headers and footers aren't an issue, since they usually don't contain much of the sites true content.
If you change a lot of content on it, it will majorly change your ranking. Your content is probably the biggest factor in ranking. I don't think there is a very strong link between domain name and content, except when your domain has a keyword in it for the most part.
For PPC, I'm not sure about if it helps your ranking. Found an article here