What drives value, what walks with the owner, where bodies are buried

I am considering buying a small business, a local HVAC company doing about $4M a year, and the seller has given me his numbers and his story. Before I put in an offer I need a hard-nosed diligence read. Look at what actually drives value in a business like this, what the risks are, what questions the numbers should raise, how much of the revenue walks out the door if the owner leaves, and where the bodies are usually buried in these deals. Then give me a valuation framework, a list of exactly what to demand from the seller, and the red flags that would make me walk. Visual, sharp, and honest that this is analysis to inform my decision, not financial advice.