This sounds great, but pricey: http://9to5mac.com/2015/12/10/tardisk-pear-review/
Anything in the SD slot will prevent the laptop from entering standby mode. See https://support.apple.com/en-us/HT202124
I got the 2013 11" i7/8gb/512gb for the same reason, so I could have my whole iTunes library. Problem is, my library has grown to over 1TB since then. A little over a year ago it became obvious that I was going to outgrow any kind of SSD, so I got the Mini as an iTunes server and freed up lots of room on the MBA. I keep the local iTunes library on the MBA pretty small most of the time, but have loaded it up a couple times while travelling.
I found this article useful regarding moving your library to an external device. http://www.ilounge.com/index.php/articles/comments/moving-your-itunes-library-to-a-new-hard-drive/ Just be careful, because depending on your configuration, it's possible to corrupt your library if you start iTunes and the external device isn't available.