Depends on how you file your taxes. Best case is that you will be able to both depreciate your van, tools, etc. (I think the term is "income-producing assets"), and deduct operating costs from the income stream(s) those assets produce. Worst case is it won't be deductible at all. As the newspaper tax-advice columns say, "Consult your tax adviser."
(who knows even less about tax law than he does about plumbing)