Motherboard must be designed for mining, not the typical gaming mobo.
You can get a supported processor as low as an i3 (it won't be an issue) but the higher, the fancier.
Just be cautious when building a rig with a riser, the riser must be powered.
And the 5 identical cards are good but I don't know what will happen when you add the next one.
Although they are all 1070ti-s, things can get wrong.
No it doesnt have to be designed for mining unless he wants to build a 12 or more card rig. Personally a rig with 6-8carfs in is fine. A second rig the same will always be my option as then you havent got all eggs in one basket