Look up the quickspecs for the thing, that will tell you which modules are compatible. Check ebay or homelabsales for it.
I have 8x 1GB DDR3-UDIMM ECC Modules laying around from upgrading my Dell R210 II, I would give you those for free when you pay shipping, but you're probably in the US and I'm in europe, so it's probably too expensive to ship.
Windows Desktop OS has the huge downside of forced updates these days. And they will just reboot, no matter if you want or not.
Server version is fine, you often even get it for free when you are a student for lab purposes, but it has way more overhead than Linux. 8GB is bare minimum for a Windows server, probably even more these days. If you run some services or even exchange / sql server, you need way more. A minimal Ubuntu runs fine on 512MB and Ubuntu is already quite high in memory usage compared to others. You can get away with even less of a footprint.
Home Lab is a lab environment to test out stuff and, rebuild things to test out something else.
Homeserver would be something more longlived.
Home Datacenter when you have more compute and storage in your basement than your whole town combined.
Netcup is quite cheap, but worth the price. Only thing I had issues with is how they were billing partial months. Basically if you rent a server for a day, they bill you for the whole month and then give you credit for the rest of the month if you cancel early. Hetzner does this better imo, they bill you at the end of the month.
Also backups are better with hetzner.
If you keep a device old enough to run them it’s fine