What made you determine you need two 4TB drives?
I run 6 Dahua cams, 4 record 24/7/365, 2 on motion. I get 3 - 4 weeks out of one WD Purple 4TB drive.
A midtower should have an expansion slot for the dual NIC. The NIC card requires an open PCIe slot which should be listed in the mfg's specs.