How difficult is it to install stock Android on a device which has a modified version of the OS pre-installed?

"Stock" generally means "as provided by the vendor." In this sense, the modified version of Android that comes pre-installed on these devices is "stock." A better way to think of the Play Store editions of these phones is "neat" or un-modified Android. Only when Google sells the device is this un-modified version "stock." Both the modified and un-modified versions of these devices include drivers for the specific hardware that the device contains, which allow it to function. Most drivers are proprietary/closed source, so when Google makes a Play Edition device, they typically have to get these drivers from the hardware vendor. If you want to install the "neat" version of Android on a device, you'll need to get the code from the Android Open Source Project (AOSP), compile it, and upload it to your device and hope that it boots. I would not recommend this to start out with. For the beginner, the best bet is probably to find a custom ROM based on AOSP. There are a lot of these in forums at http://xda-developers.com. Personally, I use CyanogenMod (http://cyanogenmod.org), since it attempts to remain true to AOSP, while improving it with crazy customizability. I had little issue installing it on my Samsung Galaxy S3, following the instructions on this page (but using the Odin method documented in the "Helpful Tip" section at the top): http://wiki.cyanogenmod.org/w/In... They even have an installer now, which makes the process pretty painless. Then after you get a better feel for installing pre-compiled ROMs, you can pull the source and compile it yourself and upload to your phone. CyanogenMod also has instructions on how to do this: http://wiki.cyanogenmod.org/w/De... Once you've got a handle on building stable source for a phone, then you're probably ready to dig in and build your own ROM from scratch. Granted, you can attempt to go straight to building your own ROM from scratch at the beginning; I just recommend this route as it will ease you into Android development and (hopefully) save you a bricked phone. On a side note, I was initially a bit averse to switching to Android, since I was familiar with the iPhone and had a lot of apps and music, but now that I've switched, I'm glad I did and I don't expect to be going back anytime soon at all. Happy hunting!