The best approach for collecting requirements in this scenario is to "Work with the customer to identify the requirements for utility and warranty and inform the users of what has been agreed." ITIL 4 highlights that utility (what the service does) and warranty (how the service performs) are critical to defining the value a service provides. By focusing on these aspects with the customer, the service provider can ensure that the service meets the essential needs within the budget constraints, while users are informed about the agreed functionalities.