I believe the header area could be achieved using the Page Header addon, it will also need to be a slider so I think I could add a Slider Shortcode to the page header content area?
They will also want the main content split up into columns and blocks with the Latest News block showing a list of latest articles with a thumbnail and excerpt.
Would this kind of layout/functionality be relatively simple to achieve with the theme?. In my head I imagine achieving this by using the unsemantic grid system, but would this be the best approach or would it be better to use sidebars/widgets?.
Yes, you can use a slider shortcode inside page header or even inside GP Hooks.
GP is very modular, there’s a few ways that you can use to achieve similar results.
You can use the widgets/sidebars with 3 columns (GP has two sidebar), you can create a page without sidebar and make use of unsemantic grid and maybe some extra plugins. Or more advanced, is to create a custom page template inside your child theme. It’s up to you basicaly, just keep in mind that whatever you choose, must be easy to maintain, avoiding headaches.