Upgrade Internal Hard Drive

PCIe connector in your system is a PCIe 2.0 x 4 Lane NVM Express interface. Sadly it takes a proprietary Apple SSD unit. The SSD blade drives Apple offered was either a 128 GB, 256 GB, 512 GB or 1TB.

The larger sized SSD's are hard to come by for sale on the open market, you will likely need to order a spare unit from an Apple authorized servicer, I worn you it will be expensive! The other option here is seeing if the just released 3rd party SSD from OWC will work in your system.

As for swapping out your current HD for a SSD. While that is possible you will need to get an adapter frame as your current drive is 3.5" while the SSD is 2.5". In addition your system leverages a builtin thermal sensor within the HD. You'll need to replace it with this adapter from OWC: OWC In-line Digital Thermal Sensor for Hard Drive Upgrade for 27" iMacs 2012 and Later in addition to the SATA SSD unit.

The newer Thin series systems are much harder to open and you do run the risk of damaging the display assembly if you don't follow the IFIXIT guide: iMac Intel 27" Retina 5K Display Hard Drive Replacement or if you dare: Sostituzione SSD Blade iMac Intel 27" Retina 5K Display

Remember the PCIe blade interface is much faster than the SATA III interface and the Thunderbolt II interface is also faster than the internal SATA port.

Frankly, I would not take the risk here and besides, the larger SATA SSD's are still quite expensive! I would go for an external Thunderbolt 2 RAID unit like this one: ThunderBay 4 Mini. It will offer better performance than the SATA SSD and offers less risk.