Combining Residential Units?

Hello, Need some guidance on a possible investment of a 5-plex residential unit. Interested in purchasing and then combining two of the units to make the property a 4-plex and then reselling. We are aware of the zoning/building requirements in the locations of interest. Our main question is about tenants. Should we purchase a property that is vacant for the easiest/quickest transaction? What happens if the 5-plex has tenants? Are we able to require that tenants vacate after we become the owners. If anyone knows a good book on the subject or has any knowledge in this area, I would appreciate your feedback. Many thanks...