I have been given an iMac 2011, the HDD drive died and Apple have removed it. The cost for them to put and new HDD in was around $450.00 so the owner opted for a new machine.
From reading unless you use a similar make HDD as taken out of the machine (Seagate Barracuda 7200) you will have issues with the sensor for the fans.
Using this is an option but its $70.00 plus the HDD.
I was going to go with a new Seagate HDD and try my luck, I have it working booting from an external drive but the fans are spinning up quite often.
Anybody had any experience with this iMac, getting the LCD screen out is no problem as I have done it a few times previously.