You should be able to achieve having different branding on the same site.
Instead of buying a separate site you can use/register "different domains" specific to your "themes" then when you submit your site to a search engine you use that domain name and the specific "theme main page" you want to point people to.
For example;
http://www.domaintheme1.com/theme1mainpage.htm
For your next theme you would do the sample;
http://www.domaintheme2.com/theme2mainpage.htm