On the topic of cargo space, the more of it you have, the more **** you'll bring or accumulate while on the road.
A tow car will be a must. I would take some time to chose that one carefully also. If you really think you need more space a small pickup truck, with a hard top, and tow hitch can't be beat.
Also many FT'ers ultimately end up with a semi-permanent place in the winter to hunker down at. This is where they'll have all that extra "stuff." One couple I know leave a spare car, and even a pontoon/trailer.
Since your rig will be gas and you'll more then likely be pulling a car remember gas stations can be a PITA for a longer gas rig when you're in unknown territories. That's when nimble is good.