Lets break it into several parts (and these can be separate or all on one PC pending what you want to do).
- File Store : Hosts the actual media and shares it to the Network. Needs to have plenty of SATA Ports, Bays, with good throughput. Does not need a high end CPU or GPU. Could be a PC, NAS, or Direct Attached Storage (eg box with a Sata or USB connection). From my experience many of the cheaper NAS or Attached Storage boxs have poor throughput or unreliable connections. Does not
- MC Server : Runs MC and shares the Database to MC Clients. It will also be the box that will do any of the "Transcoding" if needed by the clients. If transcoding you will need a strong CPU. I have a i7-8700K Overclocked and it can transcode all my UHD with the excpetion of Billy Lynn's Long Halftime Walk (which is 4k/60p - eg much higher frame rate than 46/24p on most discs). Hendrik is currently also looking at adding GPU Transcoding and if this happens in MC then this PC will need a strong GPU.
- MC Client / HTPC : Runs MC as a Client, and will Load the Library from the MC Server, and then play the media from the File Store. Will need a strong GPU if using ROHQ (madVR) and a reasonable CPU.
As I said you can have all or some of these on One PC, (or two, or three etc).