Hi Miranda,
Great post. Without more information, it is hard to comment effectively and of course the answer is likely to be, "It Depends."
I would offer for your consideration to not even think about organization. Rather, focus on the business problem and from that vantage point focus on the business need with respect to processes that may need to be consolidated, or fixed, or even put in place to begin with.
This work does not necessarily require a PMO. Arguably, it can and should start in the functional units first. This also will help to keep the process work at the right level vis a vis effort, value, and benefit as opposed to over-architecting. And, potentially, based upon success and the desire by the funtional units to further improve and to have a shared organization to facilitate improvement, a PMO is born.
Good luck,
Mark Perry, BOT International