Integrate a new theme for an existing Magneto site - With certain modification!

I have an existing Magento store running with a custom template but am not very satisfied with it. I want to change the them with a new one.

I have purchased a different theme and would like to integrate this new them to my existing store. Since my store is live and it has decent traffic coming in, this new theme has to be installed on a sandbox (test server) and once it is ready, it can be moved to the live server.

- I need to have all the existing extensions running exactly as it is in the existing site. There are only 2-3 extensions that are currently being used.

- I need to make sure the new template is completely done to suit my requirements. There are a few more extensions I need to be installed in the new template.

Thanks