In reply to Chopper:
I looked in to this a few years back, one of the concerns mentioned was if the GPS function worked without a network connection. Some of the apple products wouldn't. Despite the advertising, mobile coverage is not good in the UK. I'm sat in an office in the South East and none of us have any connection to any network.
I ended up with a Samsung Galaxy tablet that I still use, good GPS and no network connection required and it has SD card slots, for saving/removable data storage. It gets thrown in my travel bag and regularly used in my leaky old Land rover with no case or screen protection.
Things may have changed, but worth looking at.