G'day Sam,
We have a soft floor Camper Trailer 7 x 5 and is quiet easy to set up. Folds out sideways. With yourself at the front and the two boys at the back, lifting it should be quiet easy. My wife and I are up onb the bed and our 2 Monsters (2 & 6) sleep on their camping beds in the fold out part. The Annexe section we use for cooking, sitting etc.
As for the car, maybe find a car on dedicated gas or duel fuel. We bought ourselves a "new" car before our trip the other week. An 08 ("new" for us) BF Ford Falcon Sedan on dedicated gas. We drove from Adelaide to Tocumwal (NSW) to Bendigo (Vic) then back to Adelaide and spent just under $400 on gas for the whole trip. We travelled just over 3,500km's. On Petrol or Diesel you'd be looking around $1,000 possibly more just on Fuel.
Enjoy!
PS - As mentioned, I have a 2 and a 6 year old. Do you want to take them with you and bring them back when they grow up?....................Please? ;-)