Hi Ecobogan,
I love the idea of the lightweight buggy. I’m thinking about something similar for a kit car.
To test your numbers, 91 5Ah 25C/35C batteries give 10kWh indeed and the way you plan to stack them, 65Ah @155.4V too. Weight is nice too - about 60kg.
But, 400hp motor, or 300kW @155.4V...