The terms of the contract are as follows: A separate contract for each duplex @ $100K each, I have 90 days after the death of the second spouse ( the husband passed recently) to purchase any or all the properties.

Without the rule of law the real estate industry does not exist or all you built can be gone with the snap of someones fingers.  

Doing a seller-financing deal for multiple distressed houses. Total offer is $350,000 at this point. I want to do an installment sale contract because they all need septics, which have to be done within 6 months of ...

I can sell a little or all of my stock any time.

I'm not that familiar with an NNN lease, I just know that means the leasee pays most, or all, of the expenses.  

Or all 3?

Now, they may or may not be able to use some or all of the depreciation depending on their specific tax situation but that’s the gist of it.

@ANTONIO A.Real estate investing is better than investing in stocks/bonds from a tax perspective because depreciation will shelter some or all of the rental income.

It is difficult for attorneys to get money directly from the LLC's, but not from your assets inside those LLC's and I imagine the bulk, or all of the assets in your LLC's belong to you. 

Make sure you get the sewer line scoped, electrical systems, and galvanized pipes and plumbing or all common issues with older homes.