"and there are many different rules there."

Exactly. Read all the variables, so that you determine which apply to the scenario for this taxpayer, and that helps you see if it meets the reporting threshold.

Example: buying land adjacent to the Marina your client operates for better access and more parking spaces, and paid $600 or more interest this year to the seller = Business property.

Example: buying an empty lot from the sister-in-law because one day, I might build a home with a boat shop on it, and paid her more than $600 in interest this year = does not meet the reporting requirements.

Too many variables for strangers on the web to direct you.