Collect the executed leases with every amendment, the assignment or consent documents, current rent ledgers, deposit receipts, any guarantee signed personally, and correspondence with each landlord about renewal. Also gather bank records showing where deposit money came from and how it moved. Missing items can usually be added later, so identify gaps rather than delaying.
Trace the money behind each deposit
A closing statement showing a paid deposit is not the same as proof of lawful source and path. For each transfer, keep the originating account, the reason the funds were there, and every intermediate movement. If part of the purchase is borrowed, record what secures the loan, because collateral affects whether the money counts as the investor's own capital at risk.
Borrowing is not disqualifying by itself. Order the collection by what expires or becomes harder to obtain: landlord correspondence about a renewal is easy to get while the negotiation is live and awkward afterwards, and a bank’s retention window may close before it occurs to you to ask. Request the historical statements first and the summaries last.
One practical marker of a complete trace: for each transfer, somebody unfamiliar with the file should be able to say where the money sat before it moved, why it was there, and what it bought, without asking a question.