I have a late 27″ 2012 iMac with 16 GB memory running Catalina. I’m looking to buy a new Mac Mini or Studio and want to know if I can use my older iMac as a display. Would I be able to use the older iMac’s hard drive also? What kind of cords would I need and how would I set this up?
I want to do this to save money on a display, mouse, and keyboard. I would also like to be able to use all of my existing software and hard drive space.
The old "Target Display Mode" doesn't work with new systems.
And you really don't want to anyway. The reason you don't want to is your old display is nowhere near as good as even a cheap display today. It is a pre-retina display, which means you have 1/4 the number of pixels as a regular display today. Very poor quality by modern standards. Even a $300 4K display will beat it, and use a lot less power too.
The hard drive in it is very very slow compared to today's drives, and at 10 years old no longer something that is reliable. You can get a new drive probably much larger and definitely faster that won't cost much if you need an external drive. A portable 4TB drive is less than $100.
As for existing software, you can install anything you bought in the Mac App Store om your new Mac, unless the app is old and has been dropped. Other software vendors should also let you install, providing you are up-to-date with the apps. You won't be able to use old 32-bit apps, but every software developer worth anything has moved to 64-bit or M1-optimized by now.