Minimum legal setup? Don't even do an LLC, just bill send them invoices, and file your quarterly taxes properly. You can just deduct your expenses from your yearly taxes in April. You should keep receipts and a journal of these expenses in case you get audited.
And if you get sued all your personal assets are on the hook for the business debt. If you die, your entire estate is on the hook for any business issues not fully resolved before you passing.
If you are young and don't have dependents or an asset base these may not be big issues. If you have a family and possessions or savings, not having a formal business structure can be devastating if things don't go according to plan.