Newsflash: Buying A Property Without A House On It?

I don’t know what to do here and I’m looking for any advice. My uncle has cancer and is looking to be having to move down to Seattle to undertake chemo and be closer to a hospital. He’s looking to sell my wife and I his property.Problem is, this property is actually a barn that has been remodeled into a 3 bedroom residence. It’s 2 and a half acres, and is still considered to be raw land on the MLS/County. It has all utilities, but is connected to an unpermitted septic system that would unlikely pass inspection.We’re looking at purchasing it, living in the barn thing (which is still under construction) and building our home on the premise. My uncle has the place owner financed @ 4.5% and his term is ending next year with balloon payment of 160k being owed. He’s willing to work anything out with us that he can, just needs the property sold and a chunk of change in his pocket to assist with the medical expenses coming up. We own a few other properties but I’ve never had to deal with a situation like this (unpermitted septic, interior that needs to be flipped).I’ve contacted a few lenders about the basic stuff (203k, construction to perm, etc.) and as soon as the unpermitted septic gets brought up they’ve all said they don’t have any options.Are there any options that could either pay for the septic along with the purchase, or get the place set up to be considered single family by the county (it’s been receiving mail for the last 10 years, and “looks” like a conversion) to have it considered a single family so traditional financing would work? I’m so sorry for the long post. It’s just been too intricate for my tiny brain to comprehend. Thanks do much.