I don’t see any attachment on your post at all, so all we know is that you have an iMac.
I have been running Unreal Engine just fine on a 2012 Macbook Pro and 2013 Mac Pro. If your iMac is anything close to those, then it should be just fine.
You can easily just install it and try running it and see how it goes. Make sure you are on at least El Capitan (preferably Sierra) and that you have run the latest OS updates first.