Hi,
I'm a Roumanian living in the South of France passionate of electronics and all that is repair or DIY.
I own SoroS Repair, a little shop specialized in Apple products repair. I mainly do Logic board level repair from iPhone to iMac but sometimes being curious I take other challenges.
Hope to be helpful on the iFixit site, I don't have the best answers but sometimes a least good answer is better than no answer.
If you use the Trancend replacement ssd ( https://amzn.to/2OkTZhj ) it comes with an adapter where you can slide the old ssd (is a bit more expensive with the adapter ( or you can find a cheaper one on eBay)
OWC has the same adapter but I see that is not sold on the iFixit store
You need the recommended one for the lid an a torx T5 for the screws inside
Rob Benton - Better be safe that sorry. Always disconnect the battery.
Here you have one http://amzn.to/2vtkPMp