Thank you for this very relevant and consequential question. Having been through similar scenarios myself, I would at least make sure to:
1. Conduct a financial sanity check by verifying 3 years of bank statements, tax filings, key customer invoices, and major expenses to confirm true revenue, margins, and cash flow.
2. Analyze customer concentration & continuity by identifying top customers, contracts, and churn risk.
3. Identify key dependencies through assessing company reliance on the founder, suppliers, or one product/channel.
4. Verify legal title by confirming ownership of assets/IP, no pending lawsuits or debts.
5. Assess operational viability through ensuring that the business can run post-transition with existing staff, systems, and licenses.
If those five boxes check out to a satisfactory degree, that would be enough to move forward for me.