@Ray Hayward
I would go with the environmental study first. The primary reason being time, you can get the study done in 30-60 days and get the bulk of the findings by then (asbestos, lead paint, underground storage tanks, etc.). This will allow a contractor to put together a decent remediation estimate (I would include a reasonable contingency since these rarely go as planned and the moment you tear something down you find something else to deal with). This will give you a go no go number on the environmental impact.
If the GC has done or is doing similar projects you can create a quick estimate using cost per SF and this will give you broad all in number. It will take the A&E about six months to create a 50%-65% design that the GC can use to develop actual pricing. You may want to explore some incentives offered by cities/counties and even lenders when dealing with this type of project. I believe Fannie or Freddie have a green development incentive that could lower financing fees.
Good luck