Self-hosting

The standby power of a desktop is not that massive. Especially when you don't run a GPU or display. That windows 8 machine you have is probably fine as-is.

An ATX PSU doesn't suck down 500 watts at all times, just because the sticker on it says it can. How much power your machine will use will depend much more on the CPU.

So just slap linux on it and have at it. If nothing else, you'll get familiar with the software involved, and be better prepared if you end up wanting to buy new hardware for this.

There's not really any lock-in with linux. Almost any piece of software you might run can be run on almost any distro you might consider using.

frankly for dipping your toes in, i'd just find a used desktop computer. you can usually find them cheap or free. ask your friends if they have an old pc kicking around. otherwise, ive found some good deals at goodwill etc. doesnt need to be a powerhouse, but its easy to upgrade ram, storage. for most applications a slowish pc wont be any kind of an issue.

i only just retired my 2011 era pc nas.

Welcome to your new ~~addiction~~ hobby.

I would get an Optiplex and drives. Run fedora server, portainer, and docker.

I recommend an ex-lease HP Prodesk. You get an ok cpu, nvme slot, physical space for an ssd and hdd, plus a pcie slot depending on form factor. As they use mainstream components the hardware is well supported in Linux.

Raspberry PIs are great little machines, but they're ARM based rather than x86, which can potentially limit your software choices. Once you've bought the PI, a decent PSU, some storage, and maybe a case the cost can also start to go up quite quickly. Another option you might want to look at is something like a refurbishd EliteDesk. You can get a decent spec for a similar price to a PI and those extras, it's x86, they run quietly, and they're upgradable if you need more horsepower in future.

Many people like cheap refurbished thinclients, and Intel CPUs 6th gen or newer are not so bad on power consumption.

A RasberryPi has the advantage that you will find a lot of easy to follow guides and if you get a NVMe hat for it to have fast database storage then Nextcloud should be relatively happy (it is a bit bloated these days).