Voice-to-text is already as good as it needs to be now, and most people barely use it because unless you are driving, a keyboard is better. I don't want others to hear what I'm searching for - or for the device to be always on, listening. A keyboard of some sort seems unavoidable.
As soon as you put a keyboard on it, then it needs a screen. As soon as you have those two things you pretty much have a phone - and why would anyone want a second device that's basically just another phone that could have just as easily been an app on their existing phone.
It'll need to android based if they want any one to use it as otherwise they'll trying to start from scratch, and why would anyone use something that doesn't have access to the 27 million existing app on the android app store. (see windows mobile)