My bus was kind of echoey while it still had the metal ceiling in it, especially without seats. That goes away quickly with a few furnishings.
I don't know where you're located, but if you have any significant cold weathe duringing the winter you're going to want to insulate so you can stay warm. And it works both ways because the insulation helps reduce AC requrements during the summer. It's a good investment as it reduced both heating and cooling expenses.
I'm surprised you haven't read about the basic foundation of building a skoolie yet. Yes you can cut corners, save money and still make your us look pretty, but it's going to cost you at some point. Insulation also does a lot for soundproofing these big metal boxes. Live in a bus for a year without insulation and you'll understand.
Carpet does do a good job of isulating the floor. It also reduces floor condensation during the winter, but I always consider it throw away carpet. .
I put in very good and expensive foam insulation. Everything else was just stuff out of the house at basically no cost. It can be done cheap because most of it is labor, which doesn't count. So I got a $800 spray foam insulation job and then installed about $300 worth of plywood on the interior. Not fancy and not expensive, but durable. There's no echo.
It's about where you want to draw the line on your build. You know if you have to have an indoor toilet and shower or not. I subscribe to the KISS theory. Fewer things to break down. Deciding about insulation is like deciding if you need a convertible bus or one with a roof. I've got to have a roof just like I have to have insulation. You don't have to have foam insulation but you do need insulation. You decide what works best for you.