Thanks for all the help so far, fellas, it's very appreciated!

Although Unity is capable of doing what you want, its support for UI layout is poor and you will spend a lot of time trying to find 3rd party assets to achieve the necessary functionality. It has a lot of functionality for cool-looking FPS UI, some minimal - and complex - support for list views, and that's about it.

Unfortunately I don't know a game engine that is suited for this task. As I understand it, the Football Manager code is based on their internal system (i.e. not an off-the-shelf engine), and took a lot of time to develop, possibly dating back to the Championship Manager days.

If I was making a game like this, I'd probably dedicate a couple of weeks to picking out the most complex screen in my design and seeing if I could create it in Unity. If not, I'd give up and make it in HTML5 and Javascript, because HTML does have a competent layout engine (as much as game developers whine about it) whereas most game engines do not.

make it in HTML5 and Javascript, because HTML does have a competent layout engine (as much as game developers whine about it) whereas most game engines do not.

I second that. Typescript and HTML5. Use AngularJS (or Angular2 if you're feeling adventurous) for all your UI needs, a raw canvas or some library like easeljs or threejs for your interactive graphics needs, SignalR to communicate with a server, ASP.NET Core on the server, a Postgres database... This could be written so quickly it'd make your head spin.