If you're getting a raid controller make sure it has an efi rom, because you will most likely en up using OVMF which is an efi firmware (im using my integrated amd raid controller, i had to dump the efi driver from memory, then convert it to a rom)
i've seen a lot of debian/ubuntu guides out there so you should be ok, as for the storage setup there tons of interesting configurations to try, personally i just passthrough my raid controler to the windows VM, so i can dual boot and virtualize the same windows install, if you have some ssd's lying around you can also use bcache backing for super fast read/write ops
Yes, its totally possible, but since you mentioned you're new to linux, you have to be prepared to spend a lot of time doing some research, i personally recommend KVM over other solutions like XEN because in my opinion its easier to set up, have a look at vfio.blogspot.com, i believe it is fedora centric but theres tons of distro specific guides out there.
You can do this with any distro, my guide is pretty much generic, there are some guide for ubuntu too: http://ubuntuforums.org/showthread.php?t=2266916 http://www.pugetsystems.com/labs/articles/Multiheaded-NVIDIA-Gaming-using-Ubuntu-14-04-KVM-585/
You can run a linux + kvm desktop on bare metal, and create a windows VM with a dedicated GPU, instead of having an hypervisor consuming resources under a virtualized desktop: take a look: https://bbs.archlinux.org/viewtopic.php?id=162768
On linux you can use KVM for this, if you had 4 GPU's you could in theory create 3 VM's each with a GPU and 1 for the host, here's a guide i wrote for archlinux
7) The fact that XEN doesnt support resetting pcie devices after a shutdown/reset, which means that for most radeon cards you have to restart the host after the first run or use some hacks (ejecting the card, etc)