Put a Text Object where you want it on the screen and set it invisible by default.
Use 'Compare two values'. If (number of collected buckets) is equal or greater than (number of required buckets), use the 'Set Visible' action for the Text object.
As for stopping the game, there's multiple ways you could do. You could disable whichever behaviors you're using to control the character, or set the time scale to 0 in 'System'. But time scale 0 will make everything stop completely.
Ideally, you should give your player a way to restart the game without having to refresh/exit the game. You could use 'System -> Restart Layout' for that.