Makes perfect sense, and be sure to get one that is at least the resolution of your iMac. If it's pre-5k gen iMac, then you want the monitor to be able to do 2K (2560x1440) natively. If you are on the 5k iMac, then you want the external monitor to do 5k (5120x2880) natively. Having mismatched resolution is going to get very annoying very quickly.
Apart from resolution, also ensure that the external monitor has an adjustable stand so that you can set it up at the same height as the iMac. That way, moving your mouse between the screens will be effortless.
At work, I'm on a 2019 MBP with 2 external monitors. The external monitors are at the same height while the laptop is lower, so when I move the mouse to the laptop from the monitor, it's not as easy as it is to move between the two monitors. But since I predominantly work on the 2 monitors and the laptop is to watch for e-mail, this is less bothersome to me.