Construct a comprehensive 10-year Discounted Cash Flow (DCF) model for a 50-unit multi-family apartment building. Your model must include the following assumptions: Purchase price of $8,000,000, 3% annual rent growth, 2% annual expense growth, and a 5% exit cap rate. Perform a sensitivity analysis on the Net Present Value (NPV) and Internal Rate of Return (IRR) by varying the discount rate between 6% and 9% and the rent growth rate between 1% and 4%. Provide a summary of which variable has the greatest impact on ROI and explain the implications for underwriting risk.