First, I assume that you hired the engineering firm that conducted the phase II. If not you should immediately retain an environmental engineering firm to represent you. If you have in fact hired a firm, you first need to explore your options and get clearer on what the exposure is. The engineering firm will have its own obligations to report the findings but you need to be educated on what the levels are and if they need to be cleaned up. Clean up requirements differ depending on the type of property involved - industrial, commercial or residential.
I am also assuming you would be leasing space not land so this may not be much of an issue but you really need to know more. My suggestion is that you work with a good environmental engineer and attorney.