That's an E-450 chassis, so not a super C in my opinion. (The term, like a "B+," is rather nebulous, but I take it to mean a unit built on a heavier duty chassis than a 450.) I would guess that the unit has very little weight capacity left for carrying stuff; you might even be overloaded if you had your ten people and some gas and some water.
I don't think the step in the middle is common; it's certainly not universal. A step up from the cab to the house part, on the other hand, is very common.
Crank out windows can be opened in the rain, but need to be closed for travel. Sliding windows don't work so well in the rain, but probably could be opened while en route (though big ones would create more ventilation than one might want). Crank out windows also offer more open area when open than sliders—only half of a slider can be open, while practically all of the crank out can be. I have a few sliders and a couple of crank outs in my motorhome. Both seem to serve their purpose.
To me, the unit you looked at doesn't seem to have a lot of space; rather, it has a lot of stuff crammed into a space barely able to contain it. The counter space in the kitchen area is pretty much non-existent, and getting around with the slide in looks quite difficult—the dinette and the couch would be nearly on top of each other, it looks, and the bunks pretty much inaccessible. Turning around in the bedroom without sitting on the bed looks tricky at any point in time.
At least there are five television sets so everyone can watch their own shows while you're visiting Yellowstone. :R