For basic hosting of stuff+storage management, TrueNAS has a highly polished product that lets you install docker containers with ease.

They have a curated collection that includes every piece of software you mentioned, plus the ability to install dockerhub images as ‘custom’ images.

Originally I started with a single Pentium 4 with 4x1.5TB disks, and it’s grown over time. Now at home I have 2 TrueNAS machines giving me 80TB of storage, and 3 HP elitedesk Minis running proxmox for general VMs.
I also have a managed switch, which lets me pipe the raw Internet into it, and deliver it to the proxmox hosts so I can run a virtual router with high-availability.

OpenZFS, which TrueNAS uses as its primary storage filesystem, has recently gained the ability to increase existing disk arrays by adding additional disks (as opposed to replacing all disks with larger ones) and this makes it even more flexible for future growth.

I will say though, that if the machine is dated and you load up ‘all the things’ in it, you might not be impressed by performance, so be sure to manage your expectations.

I also suggest that you consider making yourself a roadmap, so that you can plan out what hardware you’ll need to implement the ‘next big thing’

Also - the steamlink you mentioned - I’m not sure what you’re chasing there exactly, but if your steam rig is already in your home, the only thing you can do to improve latency is provide Ethernet to both the streaming sender and receiver.

Good luck!