I am looking at templates to purchase either within WordPress or at Theme Forest,
Here again, I'm going to direct you to the message I highlighted earlier:
Same Rules Apply – The Domain Mapping Upgrade does not enable the permission to use advertising, any kind of prohibited code, or upload additional themes or plugins. With the upgrade, your blog will still be hosted here at WordPress.com, which means that you will not have FTP access to your files and you will still be required abide by our Terms of Service.
So, if you host both your site and blog here at WordPress.com you will be able to use any of the themes displayed in the Theme Showcase and only those themes, including Premium themes. You will not be able to use themes purchased from external sources, such as Theme Forest or anywhere else. If you want to use other themes than the ones in the Theme Showcase, then you are looking to host your site elsewhere and not on WordPress.com.
Also, to be clear, if you set your site up on WordPress.com as you have described above, domain mapping for your site (hollisinteriors.net) and subdomain mapping for your blog (blog.hollisinteriors.net), those are 2 domain mapping upgrades. If you want to use the same Premium Theme on both sites, those are two Premium Theme upgrades. No upgrade purchased here will give you the ability to use advertising, plugins or restricted code or FTP access.
But, most importantly, before anything else, please confirm that you have carefully read the WordPress.com Terms of Service to ensure that your site and blog will be able to conform to them.