Buy the game.

I finally decided to buy some additional content. But the question is, how do I do it? The official store has nothing left except the gold version. On GOG, besides pirates and Necropolis, there are also various gold and deluxe versions. And they seem to have new weapons and maps, but buying these versions doesn't make sense if they are sold together with the game. So, is there a way to get all the possible extra content without buying the original game five times?
And what will happen if, for example, I activate a deluxe version on top of the current game?