For a dual port card, you will want an 8 lane PCIe 3.0 slot connected to the CPU. Almost any desktop CPU will have enough lanes since you won’t be using a graphics card. You can get by with a 4 lane slot, but you won’t be able to max out both ports bidirectionally at the same time.
I would get a recertified enterprise drive from Server Part Deals. Drives in the 12-18TB range currently have the best price per TB. Be sure to get a SATA drive if it’s going in a desktop.