Thanks for a great topic first of all,
The way i see it is its best to start off with a system in place, considering that you have to get everybody involved in its development, i think the most critical thing about starting a new business and developing a system at the same time, is how it shall appeal to everyone in the organizations, you don't want to drop a whole load of documents developed by an external party on the new guys, or they maybe defensive and low spirited.
New employees are usually enthusiastic and eager to participate and cooperate, use that with caution so you wouldn't lose it along the way, my suggestion is to have some sort of a program where everyone gets to give his ideas and share in building the system so that you wouldn't be swamped by one to one suggestions and end up displeasing everyon.