I'd rather have a humvee for the simplicity, but finding one less that 20 yrs old that hasn't been beat on by a 19 yr old or driven through the surf is difficult -- as is registering them. H1's are dot approved and much more comfy for long distance with better seats, a/c and noise insulation. As long as you maintain a humvee/h1 properly (most don't) they'll run forever and go anywhere they'll fit. Most don't know how to drive them either, as evidenced by a plethora of newby drivers looking stupid on youtube. If not properly driven or maintained, you'll need deep pockets or a mechanic on retainer. These are not trucks for the masses (only 11,000 sold to civilians) but they're easy to work on and easy to find parts/accessories for.
I do like what many here have done with their tacomas (a very dependable platform to build on) -- might be a better, more economical way to go unless you are into hummers. I also really like the mog and the pinz -- very cool and capable rides.
I'm not sure of the exact measurments but the approx interior is just over 6' wide,7 feet long and 4.5' to 6' high (the attatched pictures are a shorter version). The top is pretty heavy, but humvees have a huge cargo capacity (4 to 5k lbs). Here are some pictures that show the int as well as the ext:
