Used 4x4 for towing .

Medium sized 4x4 such as the patrol, cruiser or disco, petrol or diesel would be suitable.

Depends on it's weight. Check the specification for towing on each vehicle you consider. (Tow ball weight, gross weight and weight of entire rig.)

We are towing an 18' Bushtracker with 2.2 t tare with a Nissan Patrol 3 ltr td auto. As auto is only supposed to pull 2.5 t we would be over when tanks are full and van is loaded. The manual model with exactly the same engine can tow 3.5 t. (It is something to do with the breaking capacity of the auto.) Pulls it OK, but with the history of 3 ltr tds as you are aware is often discussed in the site, we are apprehensive.

Cruisers, Patrols, Landrovers are all popular for towing mid sized caravans, and no doubt a few others. Met a couple who tow a heavy van with a Navara and are very pleased. Reckoned it can out-pull a Cruiser.

If buying second hand and planning to go into the outback, get the gear box checked (if manual). A vehicle that has done a lot of towing (eg ex mining or farming) it can have a gearbox well worn by the time it hits the second hand market. The guy who rebuilds ours services them on his own vehicles frequently, as a bit if loose metal worn off will continue to do increased damage until all the gears go suddenly. Costly enough to get fixed, then add the towing bill, as they never fail when you are close to home.

Big-medium diesels are best. With heavy towing the fuel usage of petrols skyrockets. Diesels also give you the low-down torque you only get from a petrol at higher revs. I would go for a manual too.
Barnesy

Father in law has 2005 deisel pajero auto, and gets 12 k's to the litre at 105 k's, and has 20' tandem van, and gets around 7 - 8 k's per litre with that on and handles it well.
I would go with pajero or prado deisel for medium 4wd, should handle it easy.

Cheers pesty

Have a look at Jeep Grand Cherokee CRD Diesel, sadly they lose value a lot quicker than Bruisers etc form new, towing capacity equal to Bruiser, smaller physically, ride like a sedan, great economy, great rig.
Cheers Boroma 604.