Problem with complex UI systems

Fact: im doing an educational videogame for my thesis on informatic engineering. Doing it on Unity for Web/Windows platforms.

Design: My current design is very very similar to a very old game i found 2 days ago and now its serving as inspiration. Im talking about this: http://www.youtube.com/watch?feature=player_detailpage&v=3yYE2ZDIIu0#t=183s. Its basically a 2D sidescroller game , platform based, that interrupts the player from time to time to solve math/algebraic problems, reducing his life if he fails.

Problem: I already have the knowledge (thanks Ludum Dare, This Jam and GlobalGame Jam) to make the 2D Game, but what i've NEVER done before its the UI. What pluggin/implementation do you guys think its the correct one to make the "Problems" overlay. (Note that the overlay even has a calculator on it). NGUI? 2DToolkit? Another one i dont know??

Another Problem: How do i pause the game and music while the player solves the problem?

Thanks in advance!!!