Make sure you are good with their construction methods. All the aluminum toppers I've seen use very thin walled aluminum tubing, like what you might find in a camp or folding chair. And the side and rear walls are screwed to each other and the roof vs. welded. Just from what I've seen out there from manufacturers, I'd consider them light duty toppers and doubt they would hold up over time. Haven't seen one that could legitimately take a roof rack. If you want it insulated and paneled that might be another problem.
Get inside one and make sure you are cool with the way they make them.