Additional monitor

Yes you can connect most any aftermarket external display to your iMac.

see > Connect an external display to your iMac - Apple Support

I have run dual displays on many different Mac’s over the years and have never experienced any performance issues.

Yes, you can connect one external monitor to a 24" M1 iMac.

iMac 24-inch - Technical Specifications - Apple

"Simultaneously supports full native resolution on the built-in display at 1 billion colors and:

One external display with up to 6K resolution at 60Hz

Thunderbolt 3 digital video output

Native DisplayPort output over USB-C

VGA, HDMI, DVI, and Thunderbolt 2 output supported using adapters (sold separately)"

The USB-C (DisplayPort, Thunderbolt 3) ports are quite flexible (as the Technical Specifications, and the Apple Support document to which den.thed linked, indicate.)