Hello,


     If you want to mine only Scrypt coins,  since you don't need 4 PCI-E 16x ports and you can mine effectively with a PCI 1x port with PCI risers and also CPU "power" is not need for Scrypt mining(Almost totally not needed, you should be fine with a dual core amd), you do better:

-Buying a cheaper motherboard with a cheaper processor.
-Not buy a case otherwise your cards will run too hot. Better design a custom solution.
-Get all your r9 280x to 750 KH/s they are totally capable of doing that, undervolt the core and memory and instead of powercolor or VTX3D, buy Sapphire or ASUS.I personally prefer Sapphire. I haven't had any problems with with my 3 Sapphire r9 280x.
-Get a cheaper PSU, for 4 undervolted r9 280x a 1475w PSU is way too much.

     I think GDDR6 will not mean a significant increase in KH/s because my 7950 runs better with 1080 MHz at core/ 1250 at memory . If I increase the MHz at memory the KH/s will decrease. So, it's better to stay in a certain ratio between the core and memory speed.

     Finally, invest in GPU to get cryptos may be one of the safest way to get them because you can sell your hardware or find a another purpose for it.