Yeah it'd have to fit in the dash.
$400
So basically you want it to be just a gps, radio, and PC?
In that case, All I can think of is mounting a tablet on the dashboard, installing a GPS app, and the apps you need, and then you can use it.
Although I'm not sure about an app for android/iOS that supports OBD-ii
And also the fact you won't be able to use internet unless you get a 3G tablet.