I just upgraded to a 27 inch 2015 iMac Core i5 3.3 GHz and upgraded the memory to 24 GB with chips from OWC last year. Bought it online from Apple from the refurbished products section of the online store. Same warranty as a brand new machine. I have the 2 TB Fusion drive internally and an OWC Elite PRO Duo
https://eshop.macsales.com/item/OWC/MED3FR0GB/ configured as a JBOD with another 14 TB and a stand-alone 4 TB Media drive.