Hello,
I have checked the template. It looks simple, and possible to be converted to a Tumblr theme. One thing that makes me worried is "the the only thing that will need to be address is the layout and the ability to send the user back to the external site when they want to navigate out of the blog page." I can do the what you have asked for, however, you MUST use a subdomain for your Tumblr blog. Is it IMPOSSIBLE to combine a website with a tumblog. Regardless of that, we can discuss the project more if you feel like you are lost, and want more information
Regards,
Adam Azad