Just so you know, PSUs have an efficiency curve with generally the best efficiency at %50 wattage. So unless you are going to be running 800 watts draw most of the time, maybe get one that has a little less wattage, and possibly a higher efficiency rating for the same or less money.
You could always look in the manual or ask support if it has AMD-vi(AMD version of IOMMU). That being said, it would be shocked if it did not have support.
Having one AMD and one Nvidia is either the same, or a little better then having both the same.
That being said, I think that card(570) has the AMD PCIe function level reset(FLR) bug, so it is not a good choice to passthrough to a VM.
Yes
X570 and Ryzen 3000 still might have some issues for virtualization, I think things are mostly fixed now, but keep a look out for UEFI updates in the next couple months. You also probably will want a PCIe USB card so you can hotplug USB devices to the VM. You will want one with good support for FLR, messaging signal interrupts, and decent power delivery. See- Jack's Hardware: The Ultimate VFIO USB 3.0 Controller