Project or business updates, bank and financial statements showing capital remaining deployed, payroll registers and employment records, tax filings, and any correspondence from the sponsor about scope, schedule, or ownership changes. Add each family member's residence documents and travel records. Filing these quarterly, as they arrive, is far less work than assembling them under deadline.
Use one dated folder structure from the outset
Create folders by year and quarter, name files with the date first, and keep a one-page index listing what each contains. Back it up somewhere the family can reach if the investor is unavailable. Where a document arrives in another language, note whether a certified translation will be needed later. This is unglamorous administration, and it is the difference between a straightforward filing and an expensive scramble.
USCIS review of source of funds traces the lawful origin and path of the invested capital, which can include savings, business income, gifts, inheritance or loans, supported by a documented chain from origin to the new commercial enterprise. Source-of-funds documentation is the other underestimated item: assembling years of corporate records, obtaining a defensible valuation, reconciling accounts and preparing a coherent written explanation is professional work measured in weeks, not hours.