The 2011 27" iMac is an awesome machine and Hard Drive replacement is relatively straightforward. I thoroughly recommend a Seagate SSHD. Hybrid Drives offer improved performance over a 7200rpm HDD whilst offering higher capacity than an SSD at a lower price
https://www.seagate.com/gb/en/solutions/solid-state-hybrid/
To fit the replacement Hard Drive follow this iFixit tutorial
https://www.ifixit.com/Guide/iMac+Intel+27-Inch+EMC+2429+Hard+Drive+Replacement/7555
https://www.seagate.com/gb/en/solutions/solid-state-hybrid/
To fit the replacement Hard Drive follow this iFixit tutorial
https://www.ifixit.com/Guide/iMac+Intel+27-Inch+EMC+2429+Hard+Drive+Replacement/7555