A business plan is especially appropriate for someone unwilling to list a company for sale openly. An offering memorandum suggests you expect to sell the business within the next several months. A business plan sends no such message, and it does not have to be prepared in secret. If a buyer picks up a business plan six months after it was written, it has no special significance. This could be supplemented by a short letter with some key facts not in the plan, such as details of ownership.
Confidentiality agreements are usual before information is provided.
The protection provided is not watertight, but large organizations are wary of litigation. You probably want the confidentiality agreement to provide that, if no business combination occurs, the suitor will not offer a job to any of your employees for a period of two years. Employees in large corporations are sometimes looking for businesses to buy themselves, while masquerading as having interest on behalf of their companies. Asking such people to sign a confidentiality agreement on behalf of their employer usually requires review by colleagues, thus discouraging such deception. Be wary even after a confidentiality agreement has been signed.