Actually, not all was sound...
Guys that have not built one don't really know any more than you do, probably less.
I would buy a used, useless camper that has working appliances, and gut it. You will save a lot of money getting the RV stuff out of a used camper than buying new. You can easily spend $10k buying individual parts otherwise. Less than $2k for a used trailer that has water damage, but perfect parts.
Dont bother letting the cargo trailer builder insulate it. Don't even have them put in plywood walls, they will only make a mess of it, better to do that yourself and do it right.
In the ceiling you want at least 2 inches if not 3 of insulation, the cargo co will put in 1.
You want to isolate the steel frame so it does not conduct heat across the wall into the interior.
Put in at least 2 inches of insulation in the walls, not 1 as the cargo co will do - now you see why pointless to have them put in walls.
Everything as light as possible. the ceiling would look great using tongue n groove flooring as many van builds do but will add a LOT of weight, just use 1/8 paneling and paint it. Lightweight is good.
Unless you plan to also use it as a cargo trailer, then reconsider doing a big rear ramp door. It seems like a good idea to open it and have a big opening to look out, but if it is warm enough to do that then it is warm enough to go outside and have unlimited views. You lose a lot of storage space when u put in big doors.
The size is up to you. Consider how small you can go if that matters at all. can you sleep sideways (wall to wall) in a 6 footer? a 6.5 footer? The more narrow will be better for towing and aerodynamics and using mirrors. I would go a few feet longer and a foot more narrow if that were the options. 8 foot is also really high and you get into clearance issues under branches and whatever. Do you really need an extra 2 feet above your head in a little trailer? Consider just 6.5 ft height.
SInce u are building it you can customize it. Look at installing an air cond window unit, so you do not have the loud, inefficient RV air/con on the roof, out in the wind.
You can get a 5200BTU window unit for about $99 that will cool down a cargo trailer in minutes and will run on a tiny generator too. Or spring for a mini-split that is very efficient.
You can build the window unit into the interior of camper since you are starting scratch.
Water tanks, go big if you plan to boondock.
Gray/black tanks? Depends. Boondocking you can dump gray on dirt, and there are other options for the toilet than a black tank.
The propane water heater will be hard to get away from. Other items might make sense to re-think.
A traditional water pump, or one at the sink that is a fauct/pump?
Do you want to build a shower and take up all that space or make a temporary shower that you deploy inside or outside and use the sink faucet... That is space you will not use 23:45 of the day.
For space heater look at the diesel heaters, they will not need a lot of battery like a propane furnace.
Solar? Yes.
Battery, maybe a li-ion makes sense, maybe just a couple of golf cart batteries, lots of discussion on this...
WIndows? small or big? big eats up storage space and loses heat faster than an insulated wall.
Be sure to plan your electrical so you run all the wiring, and run enough wiring for future needs.
Cooktop? Maybe use just a portable camping stove instead of a built-in. Maybe a built-in makes sense for you. The portable ones do not simmer well at all though.
Fridge? I would go DC chest fridge for sure. Dont waste time with propane fridge.
On that, if you go alternative on everything then you will not be using as much out of that used camper, but all the hardware and pieces might still make it worth it, just have to do the math.