My Dell Venue 8 Pro came stock with 8.1 (pre-8.1 Update), I believe the Acer Iconia W3 was the only originally Windows 8.0 8" tablet.
I would say a good bet is 10-14 GB free, if I'm wrong, its because my estimate is too low if anything.
There's a thread on WIMBoot-ing the 8 Pro which can save quite some space - best if you keep all the original partitions intact.