I was planning to put a spare 120gb SSD that I had, but after spending 2 hours looking for where it went only to remember that it got used an an Intel NUC, I went out and purchased a new Adata 240gb SSD for $75 cad. it was the cheapest one that I could find of that size. Theoretically I could image the contents of my current HD over and it would fit with 10gb of free space.
I have Maverick loaded on the machine right now, and there are some odd things that it is doing like putting .DS Store files on the home screen, and random beach balls. Chances are now that I have a working SSD inside, I might look to load a clean copy of El Capitan directly onto the SSD and give that a try, leaving the Maverick on the original HD until I can thoroughly test it.
As for the instructions. with a mix of watching the videos off you-tube and reading the manual, I got it in without too much problems. I ended up doing sort of twice as the first time around i don't think that i quite got the power supply in just right and had a bit of trouble getting the wireless antenna to fit right. But once you have pulled it apart once, the second time is a breeze. The first time back together i forgot to do the bluetooth shield upgrade. So I did put it on with the second go, but managed to mess up the plastic shield bit in the process. I don't know if it's something that you really need. The two sided tape on the plastic covers is supposed to say with the plastic, but when I took mine off, as the description is not all that good and no video by OWC I ended up throwing it out only to realize afterwards that was a mistake. The mac mini books fine and the bluetooth mouse works fine so all must be OK.