Yes on October 18 when Windows 8.1 hits General Availability & you need to have a Retail key activated Windows 8 to see the link on the store
At least from what I have read around, the Windows 8.1 (Official release) won't be available on WU, but in the Windows Store this October.
Yes...it will be available from Windows Store only & KMS users will have to get an ISO to upgrade as was the case with the 8.1 Preview