The sale documents and payment schedule for anything still owed, identity and civil records for everyone who might be included, and a complete history of prior U.S. applications and admissions.
Start with the transaction, not the application
Produce the sale agreement for the land, the closing statement, the mortgage or promissory note taken back, the schedule of payments received and outstanding, and any security registered against the property. An accountant should confirm the after-tax position and whether the receivable could be sold or discounted, and at what price. Then the ordinary set: passports, birth and marriage certificates for each person who might be included. Then the history: every previous U.S. application, admission, refusal, withdrawal or overstay, with dates and outcomes, because admissibility turns on that record. Finally keep the dated copy of the official program material relied on, so that a decision made can later be traced to what was actually published at the time it was taken.
Hypothetical example: an orchard-irrigation equipment dealer has liquidity from a recent sale but keeps the program information only in an email from a broker. The first review should preserve a dated copy of the official program materials, then build separate folders for identity and civil records, immigration history, the underlying eligibility evidence, and the funds. Bank statements and transaction documents trace the payment source; passports and civil certificates establish identity and family facts. Sequence collection before any transfer, because terms and requested records may change. The usual documentation failure is treating a brochure or intermediary invoice as proof of the program's requirements. It is not. Keep the official page, the payment instructions, and evidence that the applicant made the payment directly to the U.S. government in the same file.