2012 27-inch iMac – Am I crazy?
The 2012 iMac will run Catalina natively, so you can go a little further than Mojave. If you want to try without support, people have had success with Big Sur and Monterey as well, but I can’t speak to that personally. These are great Macs and still work well if you have an SSD and upgrade the RAM (supports up to 32GB). I only replaced mine recently, and the only reason I did was because I needed a laptop. I still found it to be a fast, responsive, and enjoyable computer to use, but I had also pretty much reached its specs when it was new.
I would say that if you are considering one now, the most important thing to think about is how you are going to use it. These Macs have dedicated nVidia GPUs, so they still handle most graphics tasks decently. If your parents have a hard drive or fusion drive, you’ll definitely want to replace it. But otherwise, for normal computer use, browsing, productivity stuff, as long as the software you want to use is running on Catalina, it’s a very usable and dare I say fast Mac. Besides current OS support, it has older Wi-Fi and Bluetooth hardware, so if you’re using wireless, connections aren’t as fast. It also does not support some Mac OS features such as Sidecar or Unlock with Apple Watch because of this. And, of course, the screen is 2560×1440. It’s a great display, but it’s not the Retina 5K display that came a few generations later.
I know you said it came from your parents so I guess you’ll get it for a good price but that’s also a factor especially if it’s a lower spec model for which you would need to buy an extra SSD or RAM for upfront.