I have upgraded recently the HDD in my 13" MacBook Pro mid 2012 to an SSD, so can post first hand information... :) 1. Best... well matter of taste really, while some reported issues, but those were mostly with older models (pre 2012). I went for a Kingston SSD (which I had already). Probably I would purchase a Samsung EVO if I needed a new one. 2. Any standard SSD will fit nicely, the laptop HDD is 9.5 mm, so that is the maximum thickness. 3. I have the latest El Sierra, and TRIM can be enabled form the command line (for 3rd party SSDs). From a Terminal windows execute "sudo trimforce enable". 4. So you want to retain the data on the HDD, but remove OSX (Mac OS) from it. I personally kept the OS on the HDD as a bootable backup, so can not comment on this one. Good luck, my MacBook Pro reborn and feels like a brand new laptop. :)

What is worth in 2012 to answer this question... :) I have just replaced the DVD drive in an eMac 1.25GHz. It was not that difficult to be honest, but takes some time and effort, following carefully a Macworld 2005 article about this very topic.