There are quite a few double din chinese units around on amazon & ebay. Mostly have Android 4.2 etc..
Most will play DVD's, do sat nav, play music from various sources, bluetooth handsfree etc..
Prices start from around £150 for a 7" screen double din unit.
I suppose it's easier to fit than messing about with a tablet or large phone, and they'll do eveything the tablet would anyway.
Dunno how good the android ones are.
The mrs has an older windows CE based unit and it's pretty good, install any sat nav you want (she switches between Sygic and Igo primo) plays MP3 from various sources and also blutooth hands free, easy menu system and now you can get them from around £45 (no sat navv) or £75 (with sat nav) for a 7" double din unit.