Re: npm start
It happened to me, the auto install does not always complete successfully, then after reading lot of articles on pi magazine, found out that we need to install few packages before we go for manual or auto installation and so far it has worked for me like thrice.
After installing Raspbian follow the below link, install node.js using a package manager, follow all the steps make sure everything installs successfully.
https://github.com/cncjs/cncjs/wiki/Raspberry-Pi-Setup-Guide
Then go for manual or auto installation of magic mirror, it will get installed without any errors, so far it has worked for me.