Hi. Looks like the time has come for me to replace my Imac, was planning on getting a 2017 and having an aftermarket SSD installed to save some money (might end up not being worth it but that's one of the reasons I'm here. . .to find out). Mainly I am trying to find out WHAT specs the SSD needs to have for a 2017 27" Imac. . .there are 2.5" SSDs, "blade" SSDs, etc. Want to make sure I don't get the wrong thing. Since they will have to install the OS on this new drive. . .what would it likely get? Sierra or High Sierra? Thanks.