which offroad nav app are you using? i'm slowly building up my own collection using MOBAC to create map files for AlpineQuest, though once i have the tile cache on my Mac i can re-create the maps for Orux, BCN or whatever easily if i change which app i prefer later.
i'm not sure i understand your reasons for not wanting to just use a tablet - how is it any easier to mount a component screen than a modern slim tablet? i can tell you after having had a 5 wire resistive touchsceen in the Jeep and then changing to a tablet with capacitive multi-touch - i'd never go back!
a tablet with the right ROM and the right connections coupled with an external powered USB hub should work fine with an external HD. though i might use multiple 128gb flash drives instead with all the offroad bouncing (maybe get a tablet with a 64gb microsd for most of your content, then put each state on a 128gb flash drive and swap as needed). the biggest problem is finding tablets that have the size/specs you want AND good ROM support AND most of all that can charge while simultaneously providing a USB host connection. i think a Samsung with a custom charge/OTG combo cable might be the best bet if you did get a tab. my original XOOM works for those requirements too but it's getting old and slow by current standards.
one last thought, if you find a tablet you like but it can't charge while providing USB host, you could get a wifi HD or a wifi router with a USB drive port, then you could mount that anywhere in the vehicle and have less to clutter the dash install.
good luck with your install however you go!