Sell it later. That is what I did for all my halloween stuff, as long as they can still get their items from the mystic forge. Look into that first though.
Yes, you can stop the install and pick it up later. I had to do it in parts and had no problems.
If your install is interrupted by an outside influence, such as a power outage, then your install may become corrupted and need to start over from the beginning.
Nope The launcher will download whatever is needed on launch day or whenever they allow you to start downloading the real game. It'll be a hefty download, of course, but it wont be the full 20GB+ you'd need to download otherwise.