I have an iMac 21.5 Late 2012 (2.7GHz i5) and I was thrilled to find a vacant Apple SSD slot!
I have since installed an SSD and I am creating a Time Machine backup.
A fusion drive will then be created and the data migrated.
Also really nice to know the RAM can be upgraded to 16GB at some point :)
At first glance I was sure this iMac would not be upgradeable but it just goes to show, you should always physically check!