Dual boot iMAc
I'm retired and have a 2011 iMac, it can run Linux distros.
If you want to try some out without making any changes to your iMac, burn a live ISO CD of the distributions that you want to try (Download the Linux ISO file that you want to try and use the Mac Disk Utility app to create a bootable CD). Restart the iMac and hold down the [C] key with the CD in the drive, the iMac will boot to the CD.
An very inexpensive Linux computer is the Raspberry Pi. The latest Raspberry Pi 3 Model B is ~£30, you will need a keyboard and mouse (your iMac ones will do) and a screen with an HDMI port (a cheap TV?). The performance is not as good as your iMac, but is reasonable. If you want to try it out as a bootable CD the download link is here.
It is possible to set up a dual boot system. I have used a number of Ubuntu based distributions which seem to be OK.