Just spent $200 on new battery and install for my 2015 MBP. I wish I'd opted for the 1t model vs 512g drive, but was getting refurb and at the time they had no 1t left - wasn't sure they'd have any more (friend of mine snagged one a few weeks later!)
Hi there. You can buy a sintech adapter on amazon that will let you put a standard NVME SSD into a macbook air/retina 2013-2015 model (and maybe some others).
It costs about $20.
With that, you can choose some of the cheap, new, fast, 2TB disks and get a massive improvement in speed and storage capacity for relatively little money. Google around; there are some recommended drives.
From what I've read the 'long' version of the sintech is the best one to buy. The upgrade process is on ifixit; it's super easy. You will also need a torx screwdriver; again you can buy a 'mac fixing kit' for $5-10 on amazon etc.
