What type of things will you be using on your new Late 2012 iMac?
I just purchased a late 2012 iMac 21.5 3.1 GHz i7 with the 1TB Fusion drive.
I have 27" Late 2012 iMac with the 3.4GHz i7, 2GB 680MX GPU, and the 1TB Fusion Drive.
I bought it at the launch of it, and it is still my main Mac.
The i7 on the Late 2012 iMacs is a beast, and is still a very capable chip. The performance of the i7 on our Late 2012 iMacs are about the same of a i5 of a Mid 2017 iMac.
I am unsure of what GPU you have in your iMac, but that is probably what is aging the most. I have the 680MX, which was the top BTO option for that year, and it was a really good upgrade over the other GPUs. I am pretty sure it wasn't an option for the 21" though.
Still, it totally depends on what you do with your iMac whether the older GPU would have an impact.
Catalina doesn't run as well on a HDD or Fusion Drive as it does on an SSD.
If you were having slowing issues, I would suggest using an external SSD, but if everything is running great, no need to fix what isn't broken.
Mojave would probably run better, but again, if you are happy with Catalina, stick with that.
This is probably the weak link in your iMac. If the HDD hasn't been replaced, it is sure pretty old.
If everything seems to be running great, I wouldn't worry about it.
But.... I highly recommend on keeping a back up of your data. Better if it was a bootable back up, just in case something happens to your Fusion Drive, you can sell boot up your iMac externally for troubleshooting, restoring, or just using it as your new boot drive.
Older Mac prices are probably going to be getting a little weird with the lower priced M1 Macs, and the performance they offer.
Yes!
Getting any Mac that still gets updates from Apple and performs as well as your Late 2012 for only $200 is a great buy, imo.