Great replies, everyone.
To tackle the pricing on cheap tents like the Smitty's, we're up in Canada so it's very difficult to get them here for less than $1300. And that's for a soft shell tent, not hard shell.
So why hard shell?
It's not really for insulation, we don't camp in winter. Mostly because winter here averages -25 Celsius and can dip all the way to -45 to -50 with windchill. Not really pleasant to camp in, and in winter there's not much to do anyway unless we drive 8+ hours. We live in a pretty boring part of Canada, haha. While I've done some winter road trips before, I've never found them terribly pleasant. The hardshell is primarily for the ease of setup and take down. Just lift it up and lock the arms and you're good to go, unlock the arms and drop it down and you can get moving again. Plus, the roof of the shell gives me the ability to install a vent fan to keep condensation down in the tent. And, it's nice when it rains to have a hard roof above your head.
I like the idea of possibly doing folding hard walls, but I'm not sure if that's the path I'd want to go down. Perhaps if we tried a second one! But for now we'd like to keep the canvas walls.
As for the weight, the limit for our vehicle on the rack is 150 pounds in motion. While stationary, there isn't an "official" limit but reasonably can easily hold our weight. The weight limits are to ensure people don't put 800 pounds on their roof then roll their car into a ditch on a turn. My wife and I are less than 400 pounds combined so no worries there.
For attaching the tent canvas to the roof and 'floor', what kind of options are there? Would using a strong glue work, then reinforcing that with staples or such? I would assume that glue could bond well between tent canvas and the canvas foamie skin, but I'm not sure that staples would hold well at all or even be necessary if the glue would be strong enough.
I'm thinking just simple strip LED lighting around the roof. I'll put a battery in the car, and run a very basic solar setup so we can run the lights, fan, and have a plug to charge phones/cameras.
I'm a terrible artist, but I've been doodling a rough plan. Once it's more legible I'll post it here. I'm not quite sure when the actual build will start, we don't have a garage and currently are in the bitter depths of winter.