There are hundreds of different headunit's to choose from. The double din fascia's for the K12 are easily bought on Ebay if you don't already have one. Double or single I would go for a HU with "Android Auto" which is not to be confused with the various android head units which are HU's running the android OS. Android Auto uses your phone gives direct access to Spotify, Google maps, Waze etc, and much to my surprise it actually works perfectly, . Android Auto seems to be standardised to an extent, so if a HU runs it, it may be a bit faster on a more expensive HU but essentially you will get the same results. How much you want to spend is up to you, I would buy decent second hand personally.
Some Android Auto head units will need to be connected physically every time you get in the car, some connect over wifi or whatever. For the Iphone people, generally Android Auto HU's will also have Apple carplay which I assume is broadly a similar setup.
One caution, I needed to shave a bit off either side of my HU (which I thankfully bought cheap-ish used) to get it to fit in the standard double din hole. Thats all just fiddling about though.
