Brodit device mounts - seriously wonderful
Some solid ideas but not the best choices. For OBDII diagnostics, as others have mentioned, the "Torque" app is massively cheaper at 3 quid and does everything you'd ever want the iODB app to do for £60, including telling you exactly why your engine management light is on (I could have spent a hundred and fifty quid at the dealers to find out I simply had a ten quid brake switch in need of replacement).
For dashboard mounts, there really is only one best choice and that is Brodit, who will sell you a standard base/mount to fit the exact position you want on your exact dashboard on your exact car, into which screws in a variety of very slim and unobtrusive cradles to fit your exact device. For example I have the centre-mount dashboard base for my Seat Leon, plus the delightfully slim mount for my Samsung Galaxy S 4 Mini; meanwhile my wife's car has the right-hand mount for her Vauxhall Zafira, plus the larger but still svelte mount for her Samsung Galaxy Note Classic. Should we borrow each other's cars, we can easily swap the mounts over. And they are a billion squllion times less bulky than any generic mount.
For offline GPS, I'm quite happy with CoPilot at twenty-five quid for the whole of Western Europe, but I appreciate that it isn't as fully-featured - or slick - as Garmin (my old employer paid for the Garmin app on my old N95 and that was better than CoPilot; but it was also one of those "nice if you can claim it on expenses" kinds of purchases; you'd be daft to buy something so extravagant for yourself unless you were spending a *lot* of time driving abroad). I really have no idea why anyone with a half decent smartphone and a Bluetooth car radio would want a separate GPS device.
On the subject of Bluetooth car radios, basic generic non-DAB models come up in Aldi and Lidl frequently for under 50 quid, usually sporting AM/FM radio, SD card, USB port, aux-in socket and sometimes CD/MP3CD. I went for the one without a CD player - because who needs the clutter when you have SD cards? - and am mostly pleased, for the money. They're not amazing - in particular the MP3 file browsing and repeat/album play modes can be very poor, and AM reception prone to interference - but if your basic needs are Bluetooth + MP3 playback + FM/RDS and nothing fancy, they are reliable and offer great value for money.