Welcome to the forum, Ray.
Absolutely +1 on the auto, which means you're looking at a D40. The auto won't give you clutch grief and every D40 auto I've driven (my own, a Titanium, another demo model and an STX-550) has been wonderfully smooth.
I've got an 18' tandem axle ... wait, there's a pic here somewhere ...
That's the Ernest Giles Rd, in Central Australia.
Anyway, you can either get the Nissan tow pack or get it from ARB. Both will be rated for 3T, both should have 50mm hitch receiver in the back, so you can change out the towball for a weight distribution hitch, or a bow shackle recovery point.
Hauling a single-axle van - if it's full height - won't return much better fuel figures than we're getting (except your newer car uses about 1LPHK less than mine because of its extra power).
We get unloaded highway about 10.6LPHK. Unloaded city about 12-13LPHK depending on traffic. Loaded (not towing) city up to about 14LPHK. Towing on flat highways starts at about 15.5LPHK and goes up with inclines and traffic to 24LPHK but typically our average is 17LPHK towing.
We have added a Long Ranger auxiliary fuel tank, giving us a fuel load of 150L which on long hauls we supplement with 4 jerry cans giving us 230L of diesel. It's proven quite handy on long hauls!