If you will build just ONE miner and don't plan to build more, this is the configuration I would recommend, I will mention US prices since I buy there so you can have a base:

- MSI Z97 Gaming 5 Motherboard $130: A little expensive but will give you 7 PCIe slots (3 x16 + 4 x1), works great and has a led code display, great for troubleshooting.
- EVGA G2 1300w PSU $185: This modular behemoth will provide all the power you will need and then some, even the 1000w version will probably get the job done but it's always a good idea to have some headroom.
- 7x Sapphire Nitro RX 470 8gb GPUs $229 e/o: These little guys use have faster MEM so they will mine ETH and other algos just great, I own these and XFX RX 480 8gb and can barely notice any difference. Stay with the RX line GPUs if power consumption is a concern for you.
- 8gb DDR3 Crucial Ballistix RAM kit $35: Cheap, reliable RAM. 8gb will be more than enough to cover your RAM needs.
- Intel Celeron G1840 CPU $45: Cheap-functional CPU, will give you all the processing power you'll require on a budget.
- Samsung 850 250gb SSD $100: One of the best SSD brands, this one won't explode on you and will give you very fast boot times
- 7x + 7x USB risers (ver003 and/or ver006) $10e/o: Why 7x + 7x?, well, risers like to give trouble (compatibility usually) and having a miner not working because of this is a complete waste. Buy x7 from 2 different brands/sources, usually the x16 ports won't give you trouble with any risers, but if you find the miner is having trouble booting up, try the second set. You can even mix them, but try filling only the x16 slots with one brand and the x1 slots with the other.
- Used DVI-I Monitor/DVI-I cable: Check for any inexpensive monitor around that will accept DVI-I signal, you'll need that. The cable too if you don't have one lying around already.
- Ethernet cable/WiFi card: Whatever fits your connection needs best, the miner needs to be able to connect reliably to your net in order to work.
- Windows 10 OS: Definitely the best option to run 7 GPUs without issues.
- Chassis: If you can work with metal/aluminum, check the net for some interesting models. If your resources/skills/toolbox are limited, you could try a PVC build, easy and CHEAP. You could also buy a pre-built chassis for anything between $100-$250. I prefer using an open chassis for better cooling.

I hope this helps.