The NK 6.0 guide is a good place to start for your stated requirements.
If you want to continue to use the 8 TB drive you have now I would recommend shucking the one you have now and getting between 1 and 3 more 8TB drives so you can run them in a raid z1 in True NAS Scale. Otherwise you will likely want to find 2 - 4 drives of the same size and keep your 8TB external for other uses.
I would also recommend getting and NVME drive, (2 if you want to set them up Raid 1) to use as a cache drive.
Don’t sweat the network, just go with the onboard 1Gbps on whatever motherboard you get and you can always drop in a 2.5Gbps card later.
Any case that fits in your space and has room for the number of hard drives you want should work.
Good luck with the new home server build and enjoy!