Can Commerce Kickstart be used to add Drupal Commerce to an existing site?
No, you can not install an installation profile after Drupal has already been installed. What you can do is make a separate installation with Kickstart Commerce and compare configuration when you're stuck.

How hard Commerce is to install depends on you previous Drupal experience. If this is your first time around with Drupal, it will be pretty hard. But if you're already familiar with entities, views and rules, you'll be fine.