What should I select for my planned rigs of 6 Rx 480 GPU and one motherboard?

What would be a good PSU for a mining rig with Asrock BTC and 6 RX 480 cards?
EVGA 1300W is what the majority uses. I personally use Corsair RM1000X and it works flawlessly. I have my cards undervolted so they consume under 120W per card.
Should I use USB powered risers or ribbon powered risers? Powered risers or unpowered risers?
Powered Risers, since the current generation of AMD cards draw a lot of power from the PCIe slot itself. Older cards did not, and so unpowered risers worked fine on them.
Should I have one PSU for one rig or 2 PSUs for one such rig?
One PSU is usually less hassle and easier to setup and what I personally prefer, but 2 PSU's give you easier resale since its hard to sell a 1000W+ PSU. You will need to get a PSU link type cable to get two PSU's to run in tandem.
What does 80+ PSU means?
80+ is a ratings standard, it means that the PSU will at all times deliver atleast 80% power efficiency, which translates to little power loss. Most Gold rated PSU's actually deliver 93+ efficiency.
One RX 480 cards will use 150 watt each. Mobo 100 watt. So total should be 1000 watt.
Some suggest 1700 watt PSU.
Again, 1300W is easily sufficient for 6 RX480's, and even that i feel is overkill. I run them with 1000W PSU's but you need to undervolt to keep power down.
If I have a 1700 watt PSU and I only use 1300 watt, will the PSU actually use 1700 watt nevertheless?
Ofcource not. The total power consumed from the wall would be (Cards+Motherboard+CPU+SSD)/(PSU Efficiency) so for a 6 RX 480 setup at stock, this would be (900W + 30W + 40W + 10W)/(0.93) = 1054W.
Realistically with BIOS mods and undervolting, you should draw no more than 120W per card, so total power would actually be (720W + 30W+40W+10W)/(0.93) = 860W.