If you could get a WoL + HotS bundle, I suggest that you do so so you could start learning the game (we have a weekly practice group that you could join in). I checked Bnet just now and the bundle is only US$34.99 after US$5 discount.

You could wait til LotV comes and the bundle could be discounted further, but LotV may not arrive until late 2015 or early 2016.

For #3 though, download the full game first before playing it. While you could definitely play it when you've hit "Optimal", I recommend not playing yet because there is a possibility that it would corrupt your game files and you would need to reinstall/redownload again. It happened to me before, so yeh, good luck.