Interested in buying but a few questions first.

Hi there. I just have a few questions:

1) Are there computer controlled opponents available?
2) Is there a skirmish mode available to practice?
3) If I purchase the game, can I redownload it later incase I have to reformat or something? Or is it a one-time download?

Thanks. I tried looking around but couldn't find answers to these questions anywhere. This game looks really awesome and might end up getting it =|