We had a Beetle we did off road races with some years back. They are very capable. We used a light truck type tire then. I would go and see a tyre expert to see what is presently available in the market and to fit your budget. Keep in mind that the Beetle suspension can quite easily be raised at the rear to increase rear ground clearance especially if it has a bit of a load. Not too much as it tends to affect the rear wheel alignment (positive camber) if excessive. Interesting project. Keep us posted.
Peter Ella