I'll vouch some more for the D40 as a tug. We comfortably sat on 125km/h with the van behind us between Kulgera and Erldunda, she loved it - in fact at that speed, the torque converter won't unlock - you see a hill, you plant your foot and over you go.
I've also done 95km/h on dirt towing the van. Mind you, we're running OME Dakar springs all round and OME Nitrocharger Sports shocks too, which made a HUGE difference. The Dakar springs are dual-rate - unloaded the car doesn't kick your ass on speed humps, and loaded she doesn't sag to the bump stops.
D40 is wider and longer that most tugs (apart from Silverado/F250/F350) and gets better economy. Was talking to a guy hauling a Cardinal (5th Wheeler, about as tall as Centrepoint tower) with his F250 and he said his 7L V8 had no trouble towing it, but cruising around unladen the best he could get was 22LPHK. It drank 30+LPHK at moderate speeds. I can't imagine what it would be like hauling up and over mountains.
The new D40s - especially the V6 diesel - have it ALL over the competition in power and economy. Getting 7-8LPHK in a DMax/Colorado/Ranger is nothing (we've had one 550 owner here report 6.5LPHK). Trying getting any of those to get 12LPHK hauling 2.5T across north Qld - a user here with a 550 has done it.
Only thing about the 550 is the coin it takes to get one. Over $60K to start with, that's without mods - and for towing, you'd get at least this:
* HD springs up front
* Dual-rate leaf springs on the rear
* High heat-dissipation-rate shocks all around (eg Nitrocharger Sports, Tough Dog)
* HD tow hitch with WDH (Hayman Reese is my recommendation)
* Tekonsha Prodigy P3 electric brake controller
* Truck-style towing mirrors
* Reversing camera that can see the towball
That's an absolute minimum. If you're heading outback, also consider:
* Steel bullbar - ARB Deluxe bar with fog lights preferred
* Intensity driving lights - flood on the left, spot on the right
* Roo whistles
* Long range fuel tank - prefer stainless steel construction
* All-terrain tyres - BF Goodrich are excellent
* UHF radio (even Aldi has been selling these, $99 a pop) with a good mid-sized antenna
* DVD headrests for the kids (search this forum for "eonon")
* Dual battery system in the tub
* Engel or ARB refrigerator in the tub
* 4Ga or better cabling from front (protected by 30A thermal fuse & ignition-on relay) to Anderson plugs at rear
Can't think of much else at the moment. Oh, did I mention a canopy to hold the shit in?