For 6 GPUs You need special motherboard with 6 PCIe slots (like ASRock H81 Pro BTC), Celeron dual core processor G1840, 4GB RAM, used hard disc 100GB with Windows or Linux installed, single or double power supply min. 1000W. You need 1-6 GPUs (Radeon R9 series or RX series)... There is lot of electrical power (about 1200W for 6 GPUs) needed and heat produced.
Or do what I do...I scrape PC's in my neighborhood & use the good parts to build my mining rigs-mobo's/cpu's,ram,hdd's.
All I buy are PSU's & video cards
I build PC's with mobo's that have at least 1 PCIE 16x & 1 or more PCIE 1x,usually running only 2 video cards per machine.Keeps the cost of PSU's down,I can use smaller one's that are cheaper
Oh,you don't really need a case