Are these a thing yet? Should totally be a thing.

Failing that, is it worth re-purposing my old tablet as a car stereo/GPS/Internet Radio UI?

There's some sort of Android in-car doobrie coming, to match the Apple one. I have no idea how far along either one is to actually being available though.

And I'm sure Reddit/Instructables will have lots of how-tos for that, go for it!

Not entirely convinced a system stuck together with gaffa tape and hope is the best thing to use while driving :|

Looking around, it seems there's a Parrot thing based on Android, and a load of rather iffy looking no-name android stereo based things. Mixed reviews for them, but I've found that no-name head units in the past have had rather shitty sound quality.