I've been asked a bunch of times if an old iMac (outside of the range that can use Target Display Mode) can be used as a monitor for another computer. The conventional wisdom is that, no, it can't.
Then I stumbled upon this product:
ScreenRecycler
Use another Mac or PC as a second monitor
screenrecycler.en.uptodown.com
Using a combination of Ethernet and VNC, ScreenRecycler allows you to use an iMac as a *second* (NOT the primary, That is, it allows a two monitor setup.) monitor for your computer.
I was curious as to whether ScreenRecycler could be used to make an iMac be the primary monitor for another computer, so I wrote to the company and asked. According to Patrick Stein at ScreenRecycler, to set up an iMac to act as a primary display for a Mini: "you don't need ScreenRecycler for
that task, a VNC client would be all you need."
A VNC server is built into the Mac.
There are a bunch of VNC clients available for the Mac. Some of them are free.
I haven't tried this myself. If you try it, please report back.