What ever you have laying around is a great starting point.
It all comes down to what you want to spend vs what you want to host and how you want to host it.
You could build a raspberry pi docker swarm cluster and get very far. Heck, a single Pi 5 with 4gb of memory will get you on your way. Or you could use an old computer and get just as far. Or you could use a full blown rack mount server with a real IPMI. Or you could use a VPS and accomplish the same thing in the cloud.
And what if I don't have anything lying around?
A mid-range gaming build without the GPU is capable of running a full saas stack for a small company let alone an individual.
https://pcmasterrace.org/builds
Depends on what you want to do with it. To start any old free PC that you can find online is going to work for experimenting with it.
If you plan to run 24/7, buy some proper fans and undervolt/set power limits to CPU and you will be saving with noise and electricity.
> You could build a raspberry pi docker swarm cluster and get very far. Heck, a single Pi 5 with 4gb of memory will get you on your way.
No, you couldn't, and no, you wouldn't.
To build a swarm you need a lot of fiddling and tooling. Where are you keeping them? How are they all connected? What's the clustering software? How is this any better than an old PC with a few SSDs?
Raspberry Pi with any amount of RAM is an exercise in frustration: it's abysmally slow for any kind of work or experimentation.
Really, the only useful advice is to use an old PC or use a VPS or a dedicated server somewhere.