I can't speak on the Android Auto experience in the 2018 but I use it via Bluetooth and mirroring in my 2015. For the most part I use voice for everything and also rely on the text readout. For music I have music that the Google Play music app streams from my account and also music saved on my S8. Regarding the navigation with no signal you can download maps for offline use in Google maps ahead of time and Andorid auto will use that. There are some limitations with offline maps in terms of searching and how long the cache is saved. I haven't ran into a scenario where I've needed it yet. I would most likely use the Acura nav as a fallback option if needed. As far as data usage it really depends on the apps you use and how active your Google feed is. You can monitor the data usage for a month in the Android settings app and gauge what you may need from there. Hope this helps.