ominous ocelot

I would get some cheap maybe even used X86 hardware to start with. Depending on your backup needs, you might need more than one M2 or SATA port. If you plan to use it as an always on device, I would keep the power consumption in mind. A celeron N could be the way. The cpu processing power is, in my experience, not the limiting factor for a self-hosted environment. Give it lots of RAM, every virtual machine and every running service needs space.

Using arm, like a Raspberry Pi, is not bad if you can find all your Docker images and binaries, compiled for this platform. I went away from it.

There are many possible distributions and software you could use, it really depends on personal preference.

Can I have everything? Inside and outside the Venn circles!

It would be fun if they would never reach 3.15. Because of the pi joke. But semantic versioning has a say in this, too. Sadly.