A
My personal opinion is that I would hold off if I was thinking about going the aftermarket route since Google announced that they are updating their Android Auto software to operate in standalone mode directly on the device without a compatible head unit. I especially feel this way if they allow Android Auto to be installed on tablets which they don't do now.
Operating in standalone mode while connected to a head unit with Bluetooth or even an auxiliary cable could be almost as good of an experience as upgrading to a new head unit at no cost.
What's the point of running AA directly on a device without connecting it to a head unit? The whole point of Android auto is that it displays on a large screen and has access to the car's sensors which are better than a phones. If all you want to do is use the phone directly all you have to do is dash mount it and the use Google Maps and streaming audio in their normal mode.
AA's going to give you much better touch targets, and apps tailored for an in-car experience. (And even in the brief time I used it at IO, I completely agree with that intent.)