How old are we talking?
- Anything before core iX series is not recommended to be used as a server (missing instruction sets, low efficiency etc.).It could still be used for fun projects like installing gentoo or old redhat with plasma 2.
- If you have Core iX cpu (preferably 3rd gen or newer) you xan host some services, but look into c/selfhosted if you're interested in that.
- You could also experiment with Kubernetes and combine lots of bad PCs into one less bad PC.
In the end PCs are useful only if you can run useful sodtware on them, but besides nostalgia there ain't much use I see in them.
Since there are a lot of OptiPlexses with different specs I will give you a general advice for making home servers.
Use newest desktop you have and/or the one that took the least beating since you will need all the perfotmance and uptime you can get.
If you opt for used storage (like some hard drives you have), make them into RAID with redundancy (at lrast one possible drive failiure, preferably two if you can).
Also look for power efficiency, so if you have a laptop (and can add at least 2 drives in it for RAID) or a recent i3 or i5 dekstop (or even i7 if undervolted) that would be your best bet.
Also look for decent network interface card. Try to avoid 10/100mbit and look for 1gbit, though I doubt that old PCs can even push 1gbit. Also make sure that the LAN plays nice with linux.
For the OS, use something stable like debian, or if you want to thinker Alpine is fun and also really stable. Also Ubuntu Server is a solid choice.
When deploying services like a file server if you just want something that works (or at least should be easier than other options) YunoHost or CasaOS are your friends, but you can learn docker (or run without encapsulation) and nginx (or other reverse proxy I don't care).
For a file server everyone has their preference, but I use SeaFile since it is crossflatform and simple with good integration.
As I said, for any questions about selfhosting just hit c/selfhosted and ask away.