PSU / PCIe risers / 6-8 GPU build questions

Howdy miners,

I been trying to gather all the information on mining rig equipment / shopping list in order to build 8 GPU rig.
Everything seems rather clear but I'm having issues figuring out what seems to be the best way of powering GPUs, since there seem to be a lot of conflicting information in regards to that. Using molex is safe, it's not safe, use SATA, don't use SATA powered risers etc.

So, as PSU is concerned my idea was to buy 2x decent 850W(or something like that) and connect them with that "2-psu connector cable", that should cover the power need for the cards, I guess.

OK now onto questions:

• Are these risers any good? If not why not? And which ones are good. https://www.amazon.de/gp/product/B01N3UVJHM
• So let's assume that I want to connect 6-8 580s. And let's assume that all of them have 8pin extra PCIe connector. Here's my biggest dilemma... How would power these? Are PSUs designed to have 4 or more separate PCIe rails with enough amps to power 4 GPUs (or even more)? I've checked my current PSU and it doesn't seem so, but it's rather old one so I won't assume anything.
  
• Now, let's assume that all of GPUs are properly powered by their 8pin PCIe connectors. Does it really matter how those risers are powered? If it does, does it mean that those should be separate rails for each riser also?
• Does 2 PSU setup come with some sort of obvious drawback that I'm currently not aware of?

If I'm over complicating this thing can you just let me know what PSU did you buy, what risers, what extras (cable splitters and what not) to power your 6-8 GPU rig?

Thanks,
T.