The biggest missing fact is where the master record will live. If leads, area data and documents can all be edited in several systems, an integration may simply duplicate the confusion faster. Pick one authoritative home for each field, then test whether updates flow in one direction without...
What annual rent and loan-to-value are you using? Also, are management fees charged against collected rent or scheduled rent, and have you included building service charges separately from maintenance? Those details could move the result considerably. Vacancy should also include the gap between...