I have no idea how this insurance will work, or how fast it will be processed. I sincerely hope these people who have been through this heartache don't have to jump through government red tape forever to get what they deserve, but My bet is that it will be slow. Your buyers don't know how much or when they will get their settlement, right? A contract has to have a beginning and ending, or closing date. If this is handled like most insurance settlements, the proceeds will probably be made payable to the owners and the mortgage holder. The owners will get what's left over after the mortgage is paid. From your post, it sounds like your buyers don't know how much that will be. I would send your buyers first to a lender to get qualified. When I am working with buyers who are expecting a settlement of some sort and don't know how much it will be and don't know when it will be, I usually wait until the money is in the bank.
A HUD 203K loan is a FHA loan, which is a loan that is insured by HUD. Typically they have a lower down payment and more liberal debt/income ratios. A FHA loan, however usually involves repairs that need to be made to the home, so you have to keep that in mind when writing the contract,who is going to pay for repairs, and how much. The lender can tell you about that.
Good luck.
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Comments made are my opinion, and not intended to be legal advice of any kind.