I have considered installing an SSD into my 2013 21" iMac, because I went straight from the Macbook Air to the iMac - and I really miss the speed of the SSD sometimes. And I've been told that not all SSD's can be used - and it has to be PCIe. What are your advices, what SSD shall I buy for the iMac? I'm having a hard time sorting all the SSD's on the market, so I could use some help there.