Hi Craig & Maew,
In my view choice of vehicle should be based primarily on where you wish to go in Australia, & secondarily the level of comforts you want to have.

If you want to adventure 'off the beaten track' then you need 4wd. That capability alone (& the preparedness to use it) will open up approx 90% of the country in addition to that a vehicle suitable for only sealed roads will take you to.,

Of course there are compromises within the 4wd choices, some suitable for the main dirt roads (eg. Birdsville Track, Full Savannah Way, Gibb River Road etc) & others for the more adventurous tracks (Eg. Simpson Desert, Canning Stock Route etc).

Essentially if you choose to go with a 4wd, it will be vehicle size which dictates where you can or cannot take it. Length, height, width, & ground clearance (especially amount of overhang behind the rear axle).

This latter issue of size also impacts upon the level of comfort you can have. Generally more comforts mean less 4wd 'capability, but comforts are also dictated by budget. If you wish to minimise constraints about where you can go you can pay a lot of money for very capable 4wd truck based camper/motorhomes with a max length of 6 to 6.5 metres, or similar money for luxury accomodation but which can still take you to many places a 2wd sealed road vehicle wont.

4wd is not just about hardcore off road stuff, far from it. It is very much about being able to have the confidence to get that extra 500metres along the river bank, or that sandy track ie. a couple of kms away from the roadside camp area to a beautiful valley or beach. So it is about how you like to camp - privately surrounded by nature, or sociably among many other campers doing the same thing. Of course if you plan to mainly stay in Caravan parks that is not a choice you need to make.

Some will tell you that 2wd is sufficient & that they can take their 2wd vehicle over a few kms of dirt road to get to some of the better known campgrounds (eg. places like 80 mile beach or Barn Hill Station in WA) & they are right. Just depends on what YOU want.

4wd costs more, but my advice to anyone planning to tour Australia for the first time is to get it if you can afford it, in order not to limit your options. Many folk think 'but I've never had a 4wd & the thought of needing it is scary, so I'll just stick to the sealed roads - there is still plenty of choice', & that is of course a perfectly valid choice, but if you think you may be the sort of people who after a bit of experience may feel a bit frustrated driving past that interesting looking track into the forest, or along the river bank, thinking "i wish we could check that out to see what's down there' then you would be glad you had chosen 4wd. It's not about having to use that capability, but about having it to give you the confidence to 'have a look'. The number of free camp spots & beautiful places which are accessible immediately increases exponentially.

We travelled around the country for 18 months in a converted small bus, (think Toyota Coaster or similar). These are very popular for good reason. Generally 2wd , but with a higher ground clearance than most motorhomes, & more rugged. Ours took us down the Birdsville Track & many other places that most 2wd vehicles don't go & at 7 metres long was, for us a great size for two to travel in, but we did look longingly at dusty 4wd's coming from exciting sounding places where we didn't dare go. Since then we we have switched to 4wd & now wouldn't want to be without it.

We tow a Tvan, a combination hard top plus canvas very capable offroad camper trailer. It will go anywhere the car will, but I would still prefer not to tow. Another choice to make is mainly indoor or outdoor living? The Tvan is mainly outdoor, & has been our home over the past 5 years of travel, mainly in the warmer north. We, both now in our mid sixties, lust after the 'luxury' of indoor cooking & comfort, along with indoor heating to extend our travels more southward in the cooler months, (but still want to get to the sort of places we have been able to with our 4wd & Tvan). 

Length of time you expect to be travelling may also impact your choice. Personally if I were in your shoes I might choose differently for a 12 month trip around Australia than if the plan were for longer term travel. We are travelling long term & like to remain out in the bush for extended periods, so storage space requirements are greater than if we were on a 12 month trip & moving on every few days.  My recommendations  for  a '12 month trip' FWIW are that you check out two possibilities. Neither may appeal but both are popular choices for good reason. 1. A converted Toyota Coaster Bus.  or if you want 4wd capability 2. One of the many camper conversions based on a Toyota Troop Carrier, poptop or hi top. If you intend to travel for longer & want more luxury/storage space or whatever then choices are wide & varied & you will know what suits you best. The two I have suggested are bot good compromises one favouring off road ability, & one offering reasonable comfort & with 'rough road' capability. 

Hope those few thoughts are helpful to you. The only correct choice is what is right for you, which with your experience I'm sure you already know, but hopefully I might have added an Australian perspective to your knowledge base.

Good luck with your choosing & 'Safe travels'.