I'll discuss this from the corporation's perspective, since that usually is the party that I represent. Assuming that the corporation has been properly formed and all documentation to date is in order, the basic documents that I typically prepare (there may be more) include the following:
Disclaimer: This information does not constitute legal advice and does not establish an attorney-client relationship.
If it's a simple ownership purchase, you would need your name on the share certificates. If the % of ownership is materially large, this purchase would need to be documented as adopted in the shareholders and the board of directors meeting minutes, signed off by the secretary.
If it's beyond just share purchases and you're also getting a board seat or something, that too will need to be documented in the meeting minutes. Check that the expansion fits within the articles of incorporating to avoid incompatible scenarios. (eg: X # of board seats).
I'm sure there will be a lawyer involved in the sale of an C corp, so please use them too.