Joomla noob here. I'm just looking for an extension that will facilitate adding product pages to my client's site. The site is almost done minus the product categories and articles. Will I have to redo any of my existing work to implement K2 or can it fit right into what I already have?

K2 is only a component that is added. It should fit right into your site, but will probably need customization or (overriding a bit) to suit your needs. It will replace com_content or Joomla Articles all togther or you can use both depending on your needs. If your template already supports k2 then you may not have a lot of work to do.