I'd prefer to have the foundation set up as legal entity.
I fully agree with hari here.
In fact, governance rules would likely be automatically inherited from statutes in whatever jurisdiction the foundation is established -- this would reduce the amount of work required to establish the rules as most of the generally applicable rules are already defined. I think it would also be easier to retain accounting and legal services as an official legal entity.
I have personally witnessed successful evolution of a non-profit volunteer organization in the past. From my perspective, it would not have succeeded were it not for the force of law protecting its fair operation.
Similar ground has been successfully ploughed in the past by other OSS projects. Mozilla Foundation and Ubuntu Foundation come to mind, but surely smaller ones exist too. I don't think there is any shame in copying valid parts of their structures.