1) Yes, once you own a license to the software you get full access to the source code that you can modify as you wish. The license agreement is here: http://xenforo.com/license-agreement/. There's also lots of existing add-ons and tutorials that may help you get started in the Resource Manager.
2) As above.
3) Yes you can purchase extras at a later date. When you purchase an extra, it is valid until the end of your forum license.