How Much Time Do You Have?
I was fortunate to learn 14 months in advance that my accounting firm was moving. That sounds like a long time, but we have a fairly sophisticated network with 150 people to support, and were constructing a building from the ground up. I don't know if it would have been easier or harder to move into an existing facility. I think that cabling an existing floor plan would be more difficult, but not having to make all of the design decisions based on an existing structure might have saved time. The beauty of starting with a clean piece of paper is that I got to design what I wanted, and eventually I got what I wanted. (Obviously, more time is better, but don't procrastinate.)
Make sure that your management team knows the types of situations that should involve IT, so that you get notified early. Believe me; I learned this lesson the hard way. Early in my position as department head we acquired another company. I found out along with everyone else in the company, and then had to figure out how to merge the different technologies together. I let my management team know that in future endeavors I needed to be notified in advance so that proper "due diligence" could be conducted from the information technology side of the house.