Before the purchase agreement is signed. The structure of the financing, the ownership split and the control arrangements all affect eligibility, and each is far easier to set correctly in a draft than to change after closing.
The immigration review belongs in the negotiation, not after it
By the time a deal closes, the facts that decide an E-2 application have been fixed by other people for other reasons. A seller who wants security over the assets, a partner who wants a third of the shares, an accountant who suggests an escrow arrangement that leaves funds refundable, each acts sensibly in their own frame and each can create a problem. Building the review into the negotiation costs a few weeks at a stage where changes are still free.
After closing, the practical sequence runs from irrevocable commitment of the funds, to assembling the source and path evidence, to the application itself, with processing times depending on the route. Fatima should also plan how the Bonfield log-trucking operation is managed during that period, since her attention will be needed on both sides.