Any suitanle cable will have good thick insulation anyway ( ie welding cable ), but conduit where it passes through metalwork would be an idea.

Conduit elsewhere...debatable. At least bare insulation you can easily visually check for damage. With an outer sheath whilst it may appear ok, isnt always the case. Plus the conduit will add physical size.

And yes, just ground the battery local to a good clean bare metal part of the chassis.