You cannot share Ethernet with splitters. Each device needs it’s own cable back to a switch.

Plex can only use the GPU if you haver the paid version. Those features are disabled in the free version. I run my plex server on Windows Server 2016 with an I7-3770K CPU @ 3.5GHz and 16GB RAM, with no dedicated graphics card, just the Intel on CPU graphics. I have no performance problems with transcoding videos using the free version of plex.

I really like hMailServer for Windows. Includes both transport and backend server supporting POP and IMAP clients. Supports proper encryption and SPAM filters. No longer supported but still available.