The sale agreement and closing statement for the property, evidence of how the property was originally acquired and paid for, and bank statements showing the proceeds moving into the account that funds the purchase. Then the business purchase agreement, escrow terms and the target's financial records.
The path of funds is a chain with no missing links
A reviewer follows the money backward and forward. Backward: how was the rental property bought, with what mortgage, paid from which income, and does the tax history line up? Forward: from the lawyer's trust account after closing, to the investor's bank, to the escrow agent, to the seller.
Each transfer needs a statement showing it leaving one account and arriving in the next; a summary letter from an accountant is helpful but does not replace the statements. Hypothetical example: an East Hants seller receives the proceeds, pays down a line of credit, and later redraws the same line to fund the escrow. That is documentable, but only if the line-of-credit statements and the security behind it are included, since a loan secured on the investor's own home is treated differently from one secured on the business.
Assemble the chain before signing anything so gaps can be explained while the people who remember them are still reachable.