Regarding the spare... No, you don't need to get 5 17" rims. But you can if you want.
If you ever need to use the spare though, only use it temporarily and NEVER, EVER but it on the rear.
If one of your rear tyre's blows, replace it with one of the wheels from the front, and put the space saver on the front.
This won't kill your LSD as fast as putting the space saver on the rear will.