I used to be an IT contractor for many companies, so I will speak from the service area:
I always start each with a verbal agreement, just so we can get an idea of what needs to be done. In this time, I write down what I interpret their requirements/requests to be, any questions I want to ask about certain aspects of the project, and how much time I believe I would need. Once this is done, and all questions have been answered (again, writing down all the information, or recording with a voice recorder), I can put it into a written form.
In my statements of work, I always leave some leeway on time for "unforeseen interuptions", including emergencies (personal, family, work, etc). I don't add it as a separate item, but I include it in the labour charge, as that is usually what it would come out as. If I believe I need parts, I can add an approximate cost based on past projects, or leave it blank stating that I would get prices later and get the client to confirm. Remember, your Statement of Work should have EVERYTHING that you, the contractor, and the customer want to have completed, with a list of possible costs. It's easier to ask for more money just in case up front, then to go half-way through a project and then explain why you need more.
Depending on where you live, you may need to also charge extra taxes or fees for programs such as Worker's Compensation while on site, and you should also have liability insurance against any mishaps that happen, whether your fault or not. I include this all in the statement of work, and make sure that the client and myself sign it. We both then take a copy, and we can start on the work.