Nice lsit expanda!
Yes your choice is limited once you go a van that big, as my father in law has discovered.
I have an 06 3L Patrol ST Manual, has about 61000km on it, it was the last of the Di before the CRD upgrade. Towing my old Hawk was a breeze, but an 18 foot expanda would be double the weight of the Hawk, not so sure the 3L would be up to the task long term. In saying that though, a work mate picked up a second hand 3L Patrol at auction, 18months old, cop car, 90,000km then towed a 2t expanda around the country for 12months, he raves about the Patrol, especially considering the flogged past life it had. But there are some horror storries about the 3L on the net so who knows?
I have recently been considering a Colorado or Dmax (through work) but they are just too uncomfortable for long distance touring, plus i feel like i need a confined space permit compared to the Patrol

I have really fallen in love with my Patrol and don't want to part with it.
When i was hunting a few years ago i had my heart set on a 4.2 GU Patrol or 100 series Landcruiser, but just couldn't find a good one and they were all $15k-$20k more expensive than the 3L. I settled on the 3L and
so far it has been great (plus the price difference could go towards a chev V8 diesel conversion on day......

).
I like the idea of a 1HZ 105 Landcruiser with aftermarket turbo and intercooler, i know 'Turbo Glide' in Wollongong do a great up grade package for the non-turboed 105.
I have also heard nothing but good things about the Prado and Pajero as a tow rig. No personal experience though and not sure about towing limits.
Let us know how your research goes.