Grungus:
There was a big lady who lived near me and had a vl Commodore with the front seats removed and had the controls extended to the back seat. The doors were also modified for her to get in and out easier.
Slightly off-topic from this thread, but I kind of want to see pictures of that car. I'm really curious now.
Though, more to the topic at hand and the OP's question.. I don't know what kind of budget you have in mind, or what's even really available there. But, I wonder what options there might be for full-size vans in Australia, or how viable that might be?
Think the type of vans you may expect plumbers or electricians to use, with a sliding door behind the passenger side door, and you can walk from the rear to the front pretty easily. It also has the side benefit of being far easier and safer to get in, if you had to parallel park on a busy street.
Here in America, full-size vans tend to have a LOT of interior room, even in the driver area. And, there's passenger models with rear seats.
There's also minivans, but those will be smaller. They may or may not have enough room, I don't know.
You would also likely want to favor models with an adjustable tilt steering column, if possible. I've also heard of some models with power adjustable pedals.
If you're considering the used market, I hope there's an Australian equivalent of CarFax to give you information about the car's title, like if it's salvage, flood damage, etc.
I'm sorry but, I can't imagine a Kia Soul being big enough. Unless it's like Dr. Who's TARDIS, which is bigger on the inside. For awhile I had a neighbor with a Kia Soul, and it didn't look that big.
I have a Camry, circa '01, and I know Australian models are the same on the inside, other than being RHD to suit the local market. I'm not sure if it could accommodate a 400 lb driver that well, much less 600+ but I guess you can try.
There's also the Toyota Celica and Camry Solara, which being coupes, mean a wider, larger door though I don't know if the seat will move back far enough.