My free iMac 27" (2009) core i7 2.8, 32GB, 1TB SSD High Sierra. only cost me a SSD and about an hour of time. : r/mac
2011 imac 27" 32GB - final stage of 1TB + 2TB SSD upgrade (replaced HDD & optic drive). Also cleaned out the fans and case. Running 'SSD Fan Control' & 'Macs Fan
Further Explained: Apple's iMac 2011 Model Hard Drive 'Restrictions'