Connect iMac to iMac using Sidecar

Sidecar only allows you to use an iPad as a display for a new 24" iMac.

see > Use an iPad as a second display for a Mac - Apple Support

The 2017 27" iMac can not be used as an external display for a new 24" iMac.

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

You can connect and use an external display (like the Dell) with your new 24" iMac.

have a look at > Connect an external display to your iMac - Apple Support

Note: the 2 port model supports 1 external display and the 4 port model supports 2 external displays.

see "Display Support" at > iMac (24-inch, 2024, Two ports) - Tech Specs - Apple Support

and "Display Support" at > iMac (24-inch, 2024, Four ports) - Tech Specs - Apple Support

Sidecar is only for connecting a Mac to an iPad.

The feature that would be applicable here is AirPlay to Mac. Unfortunately, your 2017 iMac is too old to support being an AirPlay Receiver.

Continuity features and requirements for Apple devices - Apple Support

There may be third party applications that would let you use the 2017 iMac as an AirPlay display, but they would likely use AirPlay V1 protocol and be limited to a resolution of 1920x1080.

With regards to Target Display Mode, it went away when the first 27” 5K Retina iMac came out in Late 2014.

No Retina iMac supports being used as a hardware monitor for any other computer or device.

As explained above, no that will not work. Two things, first Macs are computers they are NOT external displays for other devices, yes even other Macs. Second, if you want an external display for the new iMac simply purchase one, they are not terribly expensive, you can easily find one in the $100 range or you can spend as much as $1600 for VERY nice one.

No.