Use external USB 3.0 SSD with 2012 iMac for the OS? So it is possible that I can get a good deal on a 2012 iMac. The only thing that scares me is the slow 1tb hard drive in it. I searched on how to replace it but it is extremely complicated because the iMac is glued together, I'm not ready to go down that road.

So my idea was to take a SATA3 SSD, put it in a SATA USB 3.0 HDD enclosure and then connect that over USB 3.0 and install MacOS on it.

Q: Is it a good idea to run your OS on an external hard drive? Does anyone have experience with this or alternative solutions?
Yes, it's totally fine to install and run macOS on an external HDD/SSD.

Especially since USB 3 you can get the same speed as the internal drive – or even faster in the case of an SSD.

The only downside is the fact that you will have one USB-port permanently occupied. It may also not be as pleasing to one's eye if there's an external drive permanently attached.

Enjoy your new iMac!