A quick update - I've looked into the back-end sections of the site and run some tests, asked Trish a number of questions and have figured out the best way to go about building the site that you want.
Since we're updating a live site and we don't want the site to be in state of disarray while the new theme and content is being installed and created etc., the ideal thing to do is to create a copy of all the site's files and database to what is called a "staging server", and then to build the new version there. Once complete it will replace the live site.
This will enable me to convert the existing site to the new theme, and then fit the existing and new content into it without disturbing the current live site.
Because the site was created some time ago with a custom theme (Artisteer), and since then WordPress has been updated many times, there are some technical issues to overcome to pull this off smoothly.