If you haven't bought your system yet I would recommend you get one with the largest SSD Apple offers that you can afford and then if you want to add later a second SSD or HDD you'll have an easier time adding the SATA III (6.0 Gb/s) drive than trying to gain access to the custom PCIe/NVMe SSD blade drive slot on the back side of the logic board.
If you've already bought your system then I would recommend you forgo the internal blade SSD for now. Instead get an external Thunderbolt2 SSD RAID drive setup it will be cheaper and offer as good performance as the internal blade SSD. And lastly, be a lot less work and less risk as these systems are not easy to open!
Review this IFIXIT guide: iMac Intel 27" Retina 5K Display SSD Replacement. You'll need the special tools to open the system and Adhesive strips to put your system back together. The biggest risk is damaging the display in the process of removing it, that would be a very expensive Opps!
OK, you still want to go for it! What is the SSD I need. You'll need either a two or four lane PCIe/NVMe Apple custom SSD. I would strongly recommend getting the four lane but they are much harder to find. The part you found is a two lane model (mz jpus12t/0a6) while it will work you won't get the same performance as what Apple uses in this system a four lane SSD. So far no 3rd party offers blade SSD's for this series. They also only offer two lane versions as well for the older systems.
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