Several years of financial statements and tax filings, payroll and employee records, the lease and any assignment terms, supplier and customer contracts, licence and permit details, and a schedule of liabilities. Ask how the purchase price was reached. Anything the seller will not release is itself information worth weighing before signing.
Keep two document sets, not one
Build one file for the acquisition and another for the startup, each with its own budget, evidence list and open questions. Blending them produces a plan that describes neither accurately. In both files, trace the money: personal source of funds, transfer into the business, and what it bought.
Where a lease assignment, licence transfer or landlord consent is still outstanding, record it as a condition with an owner and a date rather than an assumption in the narrative. Ask for the seller's material early and note what arrives late as well as what arrives at all, because the pattern of disclosure is itself information. A seller who produces filed accounts promptly and hesitates over the payroll register has told you where to look.
Where records are held by an accountant rather than the seller, ask for a direct release, since a summary prepared by an intermediary answers fewer questions than the underlying documents. Keep a dated log of requests and responses, which is useful commercially and later explains any gap in the evidence.