iMac (Retina 5K, 27-inch, Late 2014)

Your iMac doesn't have Target Display Mode. That went away when the first 27" 5K Retina iMac – yours – came out in Late 2014. Apple also added restrictions such that no modern Mac supports using any iMac (even an old one) as a Target Display.

As far as standalone 27" 5K monitors, you basically have three choices: the Apple Studio Display, a LG 5K display, and a Samsung 5K display. The Apple one goes for ~$1600. The LG and Samsung ones list for well over $1000 – but can be had for street prices of around $800 to $900.

27" - 32" 4K displays are cheaper – often much cheaper – but you probably do give up some sharpness compared with a 27" 5K Retina iMac screen or 27" 5K Apple Studio Display screen.

There is a third-party workaround product called Luna Display which might let you use the 27" iMac display as a second display. (IMHO, you would still need a real hardware monitor, both to set things up – and because of the limitations of Luna Display.)

The vendors of Luna Display say this about their product:

"Like any product, Luna Display does have limitations and it’s not recommended for high-motion content. For example, I wouldn’t recommend using Luna Display for gaming or video editing.

Since Luna Display is squeezing a 5K display through a USB cable, when lots of motion occurs on-screen (like from video playback or moving a window quickly) the screen can temporarily become pixelated before it renders in at full resolution."

The boldface here is theirs, not mine.

iturnock wrote:

So the question is can I connect a Mac Mini and use my old iMac as a monitor only?

No. You will need a monitor.

Sadly what you’re looking for; ‘Target Display Mode’ doesn’t work on any Apple Silicon Macs, anything with a T2 chip in it, or anything more recent than MacOS Catalina. It’s a bit of a shame, and it’s pretty obvious why it’s been implemented.