I can't speak for hardware too much as i haven't done that much research on it. But on the software front Personal eye system(
http://pes.milsistemika.com/ ) is really brilliant - and its android based, meaning alot of options when it comes to tablets.
It can download and store offline maps from google maps - and has the option to use custom maps as well.
Outside that it offers alot of functions expected from a military grade BMS system - everything from friendly and enemy positions, waypoints etc - all the way up to sharing your position or even your entire situation map through SMS or a network. (the SMS function is brilliant, anyone with a smartphone using the app can recieve your position either at will or automatically).