Existing Website CMS Backend Revamp

Redesign the existing site with an advanced admin panel. Preference of the CMS can be finalized during the discussion stage. I will give you the url which will be used as the reference. We want to keep the same exact look and feel as the front-end but need an easier to use backend management system. One that is easy to upload content, track users and interact with users. With that being said there needs to be a easy to use yet complex backend that can allow us to upload articles, do custom coding, and add very easy plugins and mods that allow sophistication. There needs to be integration with third party software such as verisgin SSL and payment gateways. This needs to be very SEO ready and not lose any optimization that may have already be done. Web Design company with SEO background will be preferred .