Hi Hannah and the family.
Decisions, decisions!!!!!!!!
If all 3 Kedrons are well looked after, any of them will take you where you want to go, within reason and common sence. The 2012 ATV would be my choice.
Weight is an issue for all caravanners, particularly for those of us with off-road vans, beit a Kedron or whatever. Quite a few owners have gone down the path of getting their vans replated and have an ATM in excess of the normal 3500kg capacity. With your Landcruiser, Lovell Springs have a GVM upgrade to 3800kg, and a braked towing capacity increase to 4000kg. Considering the age of the vans you mention, their unknown history and build status, if you want to get the Kedron replated, I suggest you contact the factory before you purchase it to see if it can be done.
There is a lot to consider when purchasing a caravan and preparing to travel. The weight issue is definitely something to consider. Particularly considering all the 'stuff' the 4 of you need to pack in the van for a year long trip, and the extra 'stuff' you will probably collect along the way. But it can be done, and that's part of the fun.
It's great you are considering a gap year to travel this beautiful country and to go off exploring those many special places this country has to offer.
All the best, and happy and safe travels.
Peter