Hi there,
I'm not an expert, and I am not sure what exactly you are asking, but I'll answer what I can
First off, the motherboard should be fine for 5 GPUs - it does have 7 slots. However, you will need risers (extender cables) for the cards, as you would not run 5 cards plugged directly into the board. For all but the first card, you will need powered risers. Also note that you would need either 16x-16x or 1x-16x risers depending on the slots you are using. Beyond that, I cannot give more detail because I am simply not experienced enough. Plenty of experts on the forums to help you though.
[From the specs on the board, it looks like you'd be using 3 PCI16x slots, and 2 PCI1x slots. So you would need 3 x 16x-16x PCIe risers, and 2 x 1x-16x PCIe risers. At least 4 of these should be powered. They are hard to get sometimes, so asking here on the forums for a supplier is your best bet]
Worth noting is that it can be tricky to get 5 GPUs working in Windows, so again you may need to seek assistance with that.
The cards are good, nothing wrong there. RAM is also fine.
My issue here is your PSUs. They are (not to put too fine a point on it) crap. The brand is fine, but they're Bronze rated, so they're not as efficient as you would want them to be. You should really investigate Gold or Platinum rated PSUs. Also bear in mind that you will need a way to connect the 2 PSUs together. The last thing would be that I am not sure 1500W of combined power would be enough here. Assuming a R9-280X can pull 280W peak (it's a bit less I think actually), you should allow 1400W for GPUs alone. I'd personally play it safe and go with 2 x 850W, or even better 2 x 1000W. Remember that PSUs only pull what they need, so a higher watt PSU does not necessarily mean a higher electricity bill.
OS is purely your personal preference. If you go with Windows, you'll need a hard disk. If you go with Linux, then not necessarily, as you can boot from a USB drive.
You didn't mention a CPU, but I am assuming that you are taking into account that you do need one?
Lastly, this link might help you:
http://www.cryptobadger.com/build-your-own-litecoin-mining-rig/Hope this helps!
Rit./
