Re: compatable stereo for cortana
Works on my stock stereo, and on anything I've synched to. The car stereo needs to properly support the handset codec the phone uses, but there's nothing special for Windows Phone. The big thing you want that gets overlooked is a way for the stereo to send a "use voice control" command to the phone so you can trigger Cortana while driving.
You can totally test it out in stores, just have a friend handy to text you.
Oh, on some stereos you need to go through a specific menu to pair for Phone usage, and another for streaming audio. My stocker is like that, so it's something to look out for.
Oh yea, and then there's the obvious. Make sure it's set to read texts out loud when connected to bluetooth.