Well you email the site where you want a link form and then you negotiate. Always check if the PR is real ! Always!
Basically, you are buying an advertisement from a web site. Paid directories, paid articles, or just a simple link on a website is called buying a link. If you are buying a link from an unrelated website to yours (unless sit is a popular high pr directory) wont help much. Try to get links from related sites that are similar to yours. That for sure will help more.
The authority links like edu,gov are best for seo.you can also try the niche related links high PR links
General insight: we all know here about buying and selling links and we're assuming that this conversation is relatively private. With that said dealing in links is frowned upon and can lead to your Website being banned by Google. Look into text ads, in effect that's exactly the same if not better. Use your preferred keyword as a title. Yes, the Site from which you're buying has to be subject-related to yours. Yes, you have to make sure that it has good ranking. Good luck!
It doesn't optimize your Site, it helps improve your Site's search engine ranking. These are two different things. Both are necessary.
It does help your reputation while optimizing mostly involves on page optimization which is modifying title, meta tags, site structure, site linking and body of the page for the targeted keyword.