The specs for a PC for a multi-camera system are below.
The capture card and External enclosure would only be needed if you are using the SDI connection for the cameras. If using NDI or other network feeds that would not be needed. Keep in mind the processing is done on the dedicated RAM on the video card, so a good video card is paramount.
Running the software needed to pull these in can take up a lot of CPU processing power as well, so make sure you have a good processor.