Assuming the lagoon is fine (and you have the reserves to remediate it + replace with a plant), I would not walk. Again, if it hits the targets on day 1 under this assumption, then there is no reason to walk it. If you aren't sure if it does, then you know what path you need to walk in diligence.
What you need to do is educate yourself on wastewater treatment fully. I am not, which is why I enrolled in the wastewater treatment engineering program at my local community college for the fall semester. We come across many scenarios like what you are talking about and I have a hard time believing that if the price is right, you cannot make it work. Education will trump fear so get educated if you aren't already.