I see that the per-requisites list 16.04. Does that mean it will not run on 14.04?
You can run it on 14.04 but you need to upgrade PHP to 5.6 from some 3rdparty PPAs.
I've upgraded to PHP 5.6 but during the upgrade of Owncloud I get this error:
Failed to fetch http://download.owncloud.org/download/repositories/stable/Ubuntu_14.04/./all/owncloud_10.0.0-1.1_all.deb 404 Not Found [IP: 188.8.131.52 80]
What is the correct repository I should use under my scenario (using ubuntu 14.04)?
There are currently some known issues with the repositories  where you can just wait until some one is fixing the issue.
Personally i would also wait for one of the next oC X releases (10.0.1 or 10.0.2) before upgrading a productive system.
Ok, thank you. I think that is what I will do.
Hope the repo maintainer has fixed the repositories until then. As i have read the next maintenance release will be available in 3-4 weeks so this is hopefully enough time to fix the repo.