roofing material suggestions - flat commercial roof

I need to replace a roof (complete tear-down) on a 40,000 square foot commercial building on Long Island in New York State. The building is one story high (about 20 feet from ground to roof) and has a large number of mechanical units and roof penetrations which will require flashing. There is no parapet but there are 6 inch roof curbs around part of the building perimeter.

I am the owner/operator of this building so I'm looking to replace this roof with a material that will stand up to abuse and will not need excessive maintenance; however, having said that I'd obviously like the roof replacement to be as cost-effective as possible. Can anyone help with a suggestion for the best possible roofing material/system within these parameters? I appreciate any help!