The Asus ZenBook Pro UX501VW can support a 2.5" SATA III hard drive... but the caveat is if the Asus comes with the 96Wh battery, then the battery occupies the space of the 2.5" bay. If the laptop has the smaller 60Wh battery, then it is possible to install a 2.5" HDD or SSD.
At the moment M.2 SSDs are limited to 512GB to the best of my knowledge. However, 1TB M.2 SSDs are just over the Horizon. I know that Samsung will be releasing the Samsung SM961 M.2 1TB SSD relatively soon. I am sure other brands will follow suit, but I do not really focus on SSD storage tech much. The SM961 will be using Samsung's newest "Polaris"controller for even better performance and I assume they will sell for $500 - $550.