It was my business to work with operators and develop a unique HACCP Plan with all associated documents and systems for their operation while providing the necessary training to ensure its success. Anyone can do it on their if they have a strong food production systems background. Finding the time is another matter.
I still encourage you to start with your local public health officials if your operation is supported by such an organization.
A consultant will get you safely functioning quicker and likely with more detail, but they are not absolutely necessary.
If you are serving from 150 to 1,000 covers a day you cannot afford to operate without a structured food safety system in place.