Willams Scotsman's parent company plans to raise $500 million in a notes offering to pay off debt connected to its pending merger with Mobile Mini Inc. Baltimore-based WillScot Corp. said Monday the senior secured notes will be due in 2025. The proceeds will be combined with funds from a $2.4 billion asset-based credit facility that WillScot will enter into at the closing of its merger. WillScot, a maker of portable offices and classrooms, plans to close a "merger of equals" with Phoenix-based…