Help for getting started with hardware

I dived into the selfhosting rabbit hole once again and again I am stuck at the hardware part. I'd like to start small-ish to make it realisable. I thought about a NAS (Openmediavault probably). First I wanted to do it on a Raspberry Pi with an external hard-drive but then I read USB connected drives are unreliable and so on. Mini PCs are too small to house internal drives so should I go with a (refurbished) business PC from ebay and add some drives to it?But they usually come with Windows 10, which I wouldn't need but makes them more expensive. I also have at least one old PC case laying around but no mainboard or CPU for it, if that info might be important. Thank you in advance for helping a noob out!

Edit: What I want to achieve: I would like a NAS and (separated) a server with some small services (pi-hole or adguard, syncthing, jellyfin (getting the data from the NAS), and so on). I thought about running the small services with docker on a RPi 4 and the NAS on a refurbished business PC with SATA drives in the case (I checked ebay and there are mainboards with 4 SATA III connectors and PCI so I could even add more SATA connectors). In a second moment a backup server (maybe with borg) would be a good idea but I could also do manual backups with an external USB HDD for the time being.