I think you have to options:
- to build a rig with maximum 3 gpu's
- build 1 rig with max 6-7 gpu's
For the first case I would recommend an onboard cpu motherboard. For it you will need only memory, ssd, 1 power supply and of course the gpus. The pros: cheaper, low consumption; cons: max 3 gpus
In the second case you need to buy a cpu too, any cpu would be good enough, the cheaper the better because the drained power is less for cheaper models. Depending on how many gpus you put you might need a second power supply.
You can see here the available motherboards:
http://cryptowizzard.com/motherboards/For the gpu part, any of the Radeon RX-470, RX-480, RX-570, RX-580 are good. For a while the 470 were performing very good because their prices were quite low. It's no longer the case because the demand was incredible and the price went up. For all the mentioned models. If you can find any of the above models for low than 250, I would go for it. For RX-580 I would go maybe a little bit over 250.
If you can not find those models on stock, or for a reasonable price, then GTX1070 would be a good alternative. You can find some of them for as low as 350. I personally think is a good price, because their performance is similar while the drained power is smaller and their price went from 500 to 350 while Rx went from less than 200 to 250 or even more.
Hopefully you can find something at reasonable prices here:
http://cryptowizzard.com/graphics-cards/
