But I wanted to hear if anyone know of such a game that already exists
There are games in which the apocalypse is happening, but it is a part of the story rather than the focus (typicaly you have to stop the apocalypse happening, à la final fantasy 7, if I recall correctly), or the prototype series which happen during a small scale virus apocalypse in Manhattan. But no complexe ai interaction in those.
Else, there is Alone in the dark 5, which happen during some kind of apocalypse. There was also a particular way of handling inventory in Alone in the Dark 5, you didn't have any menu, but rather when "opening the inventory", the character open his jacket and look inside his pockets. Is this the kind of mechanism you are looking for?
I would like to design an AI system that you can interact with basically by chatting with it, would just a complex collection of if statements be the best way to go about doing this or are there other better alternatives?
As Avalander said, the behaviour tree is an alternative (the article is very good also)
I am actually working on an AI which use the information it gathered during its life (know this place, that person, know that the player is dangerous), its mind state (happy, optimist, suspicious) and event that happened (the player threatened its family, its dog died).
But this require a really huge amount of text.