Any quality ssd would be fine, I've had great luck with Samsung 850/860. Don't worry about mm size all are 7mm and will fit..Be sure to remove the silver studs on old HHD and
reinstall on your new SSD.. Ifixit.com has great tutorials..
it is really easy on a mid-2012 MBP
PS While you have it open you may want to upgrade ram, replace battery and or fan.