You can buy a 3rd party SSD and a cheap USB 3 enclosure, plug it into the Mini, and have almost the same performance for as little as $100, plus you can use whatever size/brand SSD you want.
I would recommend going this route--buy a relatively small external SSD for your OS and programs, use the standard internal hard drive for samples and tracks.
Then try moving some samples/tracks to the SSD and see if it gives you a big speed improvement. If so, maybe you will want to move all your samples/tracks to an SSD, so you can easily buy a big and expensive SSD and switch to that.
You can also "fuse" the external SSD with the internal HDD to create a home-grown FusionDrive as well. An option to consider if you don't want to manually move stuff around....