The documents that do the work
Four of them cover most of what a small business actually needs.
Terms of trade, which set your payment terms, allocate risk, and let you register a security interest on the PPSR over goods you supply on credit. A client or supply contract that describes the scope, the price, the variations and how it ends, because most disputes are really about scope.
A lease, which for many businesses is the largest financial commitment they have, and where the rent review, assignment and make good clauses matter more than the headline rent. And, if there is more than one owner, a shareholders or partnership agreement setting out who decides what, how profits are shared, what happens if someone wants out, and how the business is valued when they do. Written when everyone is getting along, it is cheap. Written afterwards, it is a negotiation.