I decided to also replace the optical drive for a second SSD (mostly just for the heck of it, and because I never use the optical drive anyway). I used a 2TB Sandisk SSD that I got during Black Friday, as well as a 480GB OWC SSD that I got very cheaply secondhand on eBay. The OWC drives are too expensive in my opinion (particularly the "pro" version). If you shop around and/or wait for a good deal you should be able to get a 1TB SSD from a good brand for around $100.
Replacing the main hard disk is super easy. Swapping out the optical drive is a little challenging, but not too bad.
