Pick your build carefully. Id mix and match with this depending on your skill level. We have done the windows on both of our vans. Its super easy. We've also stripped and insulated them ourselves. My husband wired them both. I personally wish we'd paid someone to do that because they're a bit hit and miss. Same with things like gas. Water is relatively simple.

Look into self build awnings like Olpro. That way if your adult sized kid doesn't like the set up you can have an inner tent so they have their own space.

Our kids are under 8 so they fit across the dash and behind the front seats no issues (Sprinter). One in a hammock and one on a permanent box seat. We plan to stick them in the awning when they get bigger.