Don`t forget local occupational license for your business if you are inside a county or city that also requires them.
Also double check your zoning ordinances where you want to put the board. Would be very bad if you invested in this and then the town/city decides it is a nuisance/distraction/whatever. Make sure YOU do the checking and do not take their word for it that everything is fine. They are the ones who are gone and you are holding the bag.
You should definitely try to presell, do not take money or sign contracts with your advertisers yet. If you do not get a good response to the sales efforts, do NOT order the board. I would start with door to door sales, in the range of what you want to charge, ie, if it is expensive, do not contact your local small businesses, instead target the larger companies in your community that would benefit from the exposure.
Also ask your board salesman for references that have had the board up and running in their community for over 6 months. Will give you an idea of what to expect from your venture. If they do not have references, caveat emptor.