I'm struggling with the same thing (ease of resale vs. maximum storage/functionality).
In your case the choice seems to be based on the future of the truck. If you want to do a trial camper, mated to a truck you might sell or have your daughter drive, then stick to a standard size slide-in camper. You can still swap the standard bed for a flatbed and boxes (above and below the flatbed). When you're ready, sell the slide-in camper, flatbed and boxes, and then reinstall the original bed. All of this modularity might be more trouble than it’s worth.
The Aussies have two big advantages. First, they have a market where flatbeds are common, so resale is easier and components are readily available. Second, it’s warm. They can mount water tanks below the frame and outside of the camper box without them freezing.
If you want a long-term one-off camper that can be moved from truck to truck, I’d only consider a flatbed or chassis-mount design. In this scenario I’d stick to industry standard frame rail specs and cab-to-axle dimensions.