I mining using Msi r9 280x and asus r9 280x dc2t. Both seem to have reasonable performance around 700kh/s without any voltage modifications or bios changes.
Buy Amd sempron 145 processor
Asrock extreme 3 or asrock extreme 4 motherboard ( based on number of gpus required )
power supply - seasonic s12 or m12 series ( 300 watts per gpu + 250 watts for other components + 100w to be in safe limits and not use 100% load )
ram- 4gb corsair vegeance ( doesnt matter but peace of mind with corsair )