iMac SSD & Ram Upgrade:

Each of these 5 iMacs had an average of 100GB data, so an inexpensive 250GB SSD was used.

Thermal sensors are needed for this project, or the fans will run full RPM’s. The sensors are about $30 each.
However, in some cases, apps can control the fans but an OS upgrade could possibly render the app useless and the fans run full throttle again. Certified Apple Computer Services