The number of GPU's is not important. The number of PCIe x16 slots occupied is. Well, at least for my Sabertooth X99 that is. Using a graphic card in 3rd PCIe x16 slot means it'll work even slower or even be disabled (or disable M.2). Dunno how it is with that on Rampage V Extreme.
As for the SSD, I think currently the best option is Samsung 950 Pro NVMe.
http://www.samsung.com/global/busin...SSD/global/html/ssd950pro/specifications.html
It's currently the fastest M.2 module known to me. It isn't the largest however. Would 512 GB be enough for you? This one has insane sequential speeds, insane IOPS and Samsung seems to be one of the most reliable ones lately. I'm using a more modest Samsung 951 128GB for my HDD+SSD cache and it's jolly fast. M.2 is also bootable so you should be able to run entire system off this M.2 module alone. You can then throw out HDD cages and improve airflow even further. Rampage V Extreme only has 1x M.2 slot afaik, so that'll be your limitation. Still, if you're not a data hoarder like I am, 512GB should be fine. And a huge external HDD for movies and stuff that is useful in portable form anyway (so you can stick it to TV and watch directly from it).