Questions about new build

Hi

I am thinking about attempting to build and setup my first home server. I have done multiple PC builds in the past so have plenty of hardware experience but have never attempted a server setup.

My current purposes for the server include:
File server/media storage
Backup with redundancy
Ability to create and use VMs

However, I want flexibility for expansion in the future as my son is a gamer and may want a web server/gaming server

My options include to buy all new equipment, and if so what would people recommend? I also have an old PC build with an AMD FX8370, 32GB DDR4 and GTX 1080 I could use and just get new HDD with more storage. My current ethernet is 100Mbs with ability to inc to 400Mbs if necessary

Also what OS is recommended? windows server is expensive. I have very limited Linux experience but was considering Ubuntu

I would probably start out using your existing equipment just as a learning experience.

Open media vault is free and is a good, relatively easy to use baseline to set up a NAS.

unRAID is popular around here, and is supposed to be pretty easy to use, but does cost some money.

Proxmox and VMware esxi (free version) are both popular for virtualization, but have their own learning curves.