How important is a landlord estoppel when buying a restaurant business?

I would not waive it. An estoppel is how you learn about defaults, unpaid CAM, and whether the option is real. If the landlord will not sign, a written confirmation of term, rent, and deposit is the fallback, not silence.

We held a close once and found three months of unpaid trash CAM. The estoppel paid for itself. Build extra time into the contract rather than making it optional.