I bought one of these:
http://ca-fi.com/store/index.php?rou...product_id=409

It's basically an android tablet combined with a head unit, it will run any application that you can download from Google Play so you get a choice of navigation software, it updates whenever it connects to the home wifi. I'm using a reversing camera with it, the camera works well and the screen switches almost instantly (some brands require the head unit to fully start up before displaying the camera feed).

You can also get an optional dash cam and tyre pressure monitoring too. The dash cam is cheap and works well, the tyre pressure monitoring is expensive and I haven't tried it.