if you do the wiring from zero, you can choose a good brand of cable/wiring parts.
1 MCB group can be use for several mining rig but no more than 4.
less is better but cost you a lot of cable running down to rigs.
use branded PDU or build it your self. dont use the max capacity. always spare at about 20%

if you want to use existing wiring than you should test them with insulation tester and current leakage tester.
those equipments is also costly.

rgds,