when is apoptop suitable

Not owning a pop top, I can't help you with specific details about particular models.
Having a few friends that own pop tops, I can see your confusion, there is a big range out there. Most are in the 13' to 16' range.
Most do seem to be medium duty.
I have one friend with an old small millard, towed by a falcon, and has been around Oz at least once. They just took it slow and careful on most fo the dirt roads.
Another towed by a Discovery which is also a standard van, but has had an axle over conversion for better ground clearance in and out of creek crossings. These roads are still roads I use 2wd.
The third has an off road van, a lot more clearance, double axle, 4wd tyres and rims, worth at least $30thou.
The 4th can is a jayco off roader, towed by a 80 series. It seems reasonably sturdy, and has been to Fraser Is a few times, but not out west.
So I think the first question is how seriously will you really use the van. Not just dreams, but actual use?
Would a standard van with the previously mentioned axle over do, with care? More of those on the market, and from what I can see, as soon as you mention off road, add a good few thousand dollars.

Not sure if I a really helping, but hopefully I have a little.

IMO , poptops don't make real good offroad vans. The roof on a van provides a fair amount of stability , and because the poptops dont have a roof as such they tend to flex a bit more whcih can effect the internals.

My parents did a fair amount of touring in their Jayco poptop , and then decided to sell it and buy a proper Offroad Van. They were always having just little problems with corrigations etc.