sclater29
That is some serious build. I am sure there is no way the Prado would hold all that weight and stay within its GVM limits. I also doubt the Hilux will get in under the GVM based on my experience. Check my buildup for details.
I reckon if you stick to the Hilux, you would be better off fitting the Lovells GVM upgrade suspension kit which will increase the GVM, but this will be at the expense of the GCM sorry Brendan
The weight you can legally tow will be reduced.
If you went the tray back route, you could look at a jack off style camping back so you got your ute back when you came home.
200 litres of fuel is doable. olcoolone has 280 litres. You can get a 153 litre replacement fuel tank and another 70 litres that goes above the spare tyre. Down side of this would be the extra tank behind the rear axle which will increase the axle weight which may be a factor when you take into account the towball weight.
I did not see any water storage on your list. Not sure if that is important with a trailer behind. It is hard to do in a style side. The only trick I think you could explore with the tub is to fit your aux battery up in the underside of the tub behind the rear wheels. I am not aware of anyone doing any major hacking to the tub.
I was interested to hear feedback to the two campers I listed. If you like the Imagine, you may also like to look at the Conqueror
http://www.conqueroraustralia.com.au/ but having seen one in the flesh, it did not appeal to me. You may also like to look at offroad caravans. I have a Coromal on order which is an entry level one, avoid Jayco and I am guessing at $60k plus, there will be some resstance to the Australian Offroad and Kimberly vans. Most of the offroad vans (eg. Kedron) are too heavy for the Hilux.
Be prepared to spend at least 2 full days at a major caravan show to see all options.