Honestly, I would stay with a regular 7200rpm drive(s) for projects and audio. From my experience, a SSD doesn’t make a lot of difference unless you are streaming large sample libraries. I’d just slap in another 7200 into the Mac, unless you are totally willing to pay the premium for a large capacity SSD. If you had Thunderbolt 2 (which unfortunately you don’t), a Thunderbolt-ready SSD is incredibly fast…but VERY expensive.