If all else fails and you are not dead set on installing Mac os I've had success installing Chrome OS Flex on an old 2009 iMac and it works really well. Obviously you'll need to create a bootable USB but instructions here are good.
Edit - home use link:
https://support.google.com/chromeosflex/answer/11552529?hl=en
@goodhead started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
This site is supported almost exclusively by donations. Please consider donating a small amount regularly.
If all else fails and you are not dead set on installing Mac os I've had success installing Chrome OS Flex on an old 2009 iMac and it works really well. Obviously you'll need to create a bootable USB but instructions here are good.
Edit - home use link:
https://support.google.com/chromeosflex/answer/11552529?hl=en