The official page describes the application and vetting process; expect to provide identity documents, civil records for the family, evidence of the lawful source of the gift funds and whatever the official form requests. Confirm the current list on the official site before assembling anything.
Prepare for vetting, not for a purchase
Hypothetical example: the couple assumes a bank confirmation of the gift is the only document. The official framework describes vetting, so prepare as for any immigrant application: valid passports for each person, birth and marriage records, evidence of the lawful source of the funds used for the gift, and any records the official form or FAQ lists. Because the program routes residence through employment-based categories, further documents may be required at the petition or visa stage. Keep a dated copy of the official instructions in force when the application is made. Do not rely on an intermediary's document list; check it against the official site. Certified English translations are required for any record not in English. If a document is unavailable, note why and ask what alternative evidence the official process accepts rather than substituting something informal.
Hypothetical example: a harpsichord dealer plans to fund the payment from the sale of inventory accumulated over decades. Keep purchase invoices, sales records, tax filings, bank statements, and the transfer record that connects the lawful funds to the applicant. Add current passports, civil-status documents, and certified translations where the official process requires English. The official instructions and the source documents settle the document list; an adviser’s undated checklist cannot replace either. Make an index that identifies which documents prove identity, family relationship, source of funds, and payment. Resolve a missing invoice or inconsistent name before submitting, using an explanation and independent records rather than an unsupported declaration.