The issue is exactly how many homes will be allowed on the existing tanks. Unless you have a permit from the DNR in your hands that tells you how many you are allowed how can you even make an offer.
Call the DNR get a copy of the permit and get to the bottom of this. Sounds like part of the system is bootleged.
The real issue is the 26 homes (or more likely alot less than 26). There is not likely to be the economy of scale needed to deal with a lagoon, private well, and septic tanks.