My parents have an old Compaq Presario SR1950NX that works perfectly fine, but hasn’t been used in at least 15 years. Obviously I can’t connect it to the internet with Windows XP, but before I recycle it I thought I’d see if anyone had any cool ideas for how I could use it. I’m sure I could find Linux distro that would work, but I don’t really need another desktop PC in the house.
Using a spare system that isn’t in active use anymore to mess around and learn is definitely a good idea. However for long-term use I would keep an eye out for energy consumption. Depending on electricity prices it might make more sense to get a newer system with lower power draw, especially for something with high uptime.