Hi mechanic
We have a lagoon here in Kansas as do most of my neighbors and its the best choice for us. The lagoon is the entire septic system, all water/etc goes in there. As an aside, we almost never use the garbage disposal, there are better ways to get rid of that than sending it to the lagoon. If no one is living in the house, I would expect the lagoon level to drop. Perhaps some questions to the owners would give you more information on the lagoon.
The lagoon should be sized for the number of persons. The lagoon process has an aerobic layer over an anaerobic layer, the bad smelling layer is the anaerobic layer and it is not exposed to the air. Our lagoon is 150ft from our house and we never have any problem with odors. Good luck