I use a Samsung Galaxy Tab, I installed in the dash for now. I can use google maps etc on it when I have data. (You can also download the maps of the area you will be in for offline) I also use GPX Viewer Pro (pro cost $5 I want to say, there is a free version as well) It can pull maps from multiple sources and also has the ability to download for offline as well on a by state by state basis. It doesn't do turn by turn Nav but it does do active track right on the map so with basic skills you are more than ok. It can also graph your track on the map in case you want to know where you have been, sort of a digital bread crumb trail so to speak. It is not as elegant as some but a fantastic solution for around $100.

I have been looking at ditching the factory radio, dropping the switches on the top of the stack down to on top of the climate control switches and installing a 10-12" tablet. I have 50gig of music on a SD card and many of the newer Android tabs have a fm radio feature built in.

 

Regional maps for both MAPS.ME and CoPilot can be fully downloaded so you don't need a cellular signal to navigate. I don't know if they're on iPhone but it's worth looking.

 

I installed a Garmin GPS..
It has free lifetime updates, live traffic, backup camera, up to date points of interest and many more features.

Only catch is when you remove your current satnav, you will need a optical loop (orange cord looking thing) so the car will be happy about the satnav being removed

Loop bypass info:
WGCD 2 PCS Fiber MOST Optical Optic Loop Bypass Male Female Adapter for Benz Audi Mercedes BMW VW Porsche

Thread on GPS mount and installation is HERE

Final installation looks like this