You can transfer up to 20,000 songs from iTunes to GPM, I think the best way to do it is by googling it and seeing if any results from Google come up. I did this myself a while ago and you don't need a proper PC, a Chromebook will do.
Android L should run comfortably on an S4, as it's meant to be as processor intensive as 4.4, which was designed to work smoothly on devices with 512Mb of RAM, let alone the 2Gb on an S4.
As far as I know (I may be wrong here) the Ok Google phrase only works on the Google now launcher which I have as standard on my Nexus 5, however its easily sideloaded using an apk online. Personally I use the launcher over Nova etc. It's brilliant.
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