When you said you have your money ready I was imagining a Chinese guy with a fist full of notes ready to throw it at his screen :D (clears throat)
There's no region lock on this game. But if you are ever not able to play or activate a game we always give you a refund, no questions asked. And to your last question - When you activate the game it will say Activation successful or similar. And you will see it in your Steam library. You won't be able to change your key after activation. You can ask Steam to remove the game from your library though if you really don't want it there :p