Mac mini M2(8+10). Which older iMac model can be used as a monitor?

As mentioned by the others, the Mac mini does not support using any iMac as a display.

carefully read > Use your iMac as a display with target display mode - Apple Support

You need to get and connect an aftermarket external display to the Mac mini.

please have a look at > Connect a display to Mac mini - Apple Support

Neither unfortunately, no longer supported.

Why do you want an old iMac when you can get a perfectly good new 24" monitor for just over £100?