Imagine you're tasked with restructuring the operations of a medium-sized manufacturing company. Develop a comprehensive plan that integrates various Business Management Systems theories to optimize efficiency, quality, and productivity within the organization. Your plan should cover key aspects such as Total Quality Management (TQM), Lean Management, Six Sigma, and Business Process Reengineering (BPR). Outline the steps you would take to implement these theories, the potential challenges you might face, and how you would address them to ensure successful integration and sustainable improvements across the company.