To tackle ease of use and SEO, WordPress was a perfect solution. WordPress is the most popular content management system on the internet for a reason. The learning curve would be easy to get up to speed for their marketing staff to take over after the build. SEO and internal naming conventions had to be maintained.
On a site this big, it required a large amount of planning and checking and well as an inscrutable Quality Assurance process to make sure every page was properly linked internally as well. It was a combination of automated and manual work to verify each of the what seemed to be thousands of links.
Lastly, in order to copy the look and feel as closely as possible, we used the Headway framework due to the grid layout nature and ease of moving elements. Comparing the old and new sites, it was hard for the untrained eye to tell a difference.
We worked closely with their SEO and marketing team to ensure that the SEO would not be dramatically effected. This required a significant amount of code review and work but given the size of the site and business, this was our most mission critical element.