I am running a AMD Athlon(tm) X4 860K Quad Core Processor with 32GiB of RAM, Radeon HD 7450, 16TiB of HDD storage and 256GiB SSD. The only upgrade I am considering is buying 4TiB SSD drives to replace the HDD drives, this is only because I've noticed SSD's have gotten really cheap.

I would plan for Docker and not Virtual Machines, as VM's emulate an entire computer and then you run an entire operating system within them and then the application, the result is they need far more resources to act as a host for an application. Server applications have been moving to Docker because its a defined way to sandbox applications, run them consistently and uses far less resources.

Personally I run Debian Stable since its a home server and the only updated applications I want are Docker images and security patches. I then installed Docker Community Edition on to it.

I then deployed Portainer Community Edition on to the server, this provides a Web UI to manage the docker contaners running on the server. I have 9 docker containers currently running on the server.

You mentioned Plex: Plex provide a docker image for running their application that supports NVidia GPU Acceleration and seems to run fine on AMD hardware. You will find almost every server application offers an official docker image.

With my business hat on, think how many docker containers you want and plan for that + 1 cores in your CPU, you can probably look up the applications you want to run and add up their recommended RAM usage, as a home rule of thumb 16 GiB of RAM is the minimum, 64GiB would be overkill.