You state: “When you purchase a premium theme, it is available for the lifetime of that blog. If you would like to use the same theme across multiple blogs, you’ll have to purchase the theme for each blog.”
I want to install the whole thing on my own server
If you purchase the theme for use here on WordPress.com and later move to a self-hosted WordPress install, you cannot take the premium theme you purchased here with you. You will have to purchase it again directly from the theme author.