CPU: AMD seems to be the best bang for the buck ,just get one with enough PCIe lanes and a descent base clock speed.
Ram: I would start at 16 but 32 is a good middle ground, 64 if you like.
Storage: Samsung NVMe for me all day!
GPU: RTX3070 or 3080
SDI i/o: AJA Corvid line is solid.
I would ingest your PP computer via NDI. Your GPU will have 4 outputs. 1 for your UI. That leaves you 3 outputs for other screens.
Amazon has 300' Fiber HDMI cables for under $200, those are great.
If you need more i/o, I would slap a SDI card in your build as well.