No question that we will have the LoI reviewed by a lawyer before we sign.
My bet is you'll never see the money. Either there will be a clause hidden in the agreement that allows them to reverse the sale if their project doesn't pan out. Or they are on the brink of bankruptcy. Either way, they are structuring the deal as if they know they'll only have to pay if the project is a home run.
Don't waste your time with HN. Get a lawyer.
Your process is basically the same as having a lawyer write the code for your web app and then have a programmer review the code just before you launch. Now you are probably thinking "no that's not the same because that won't work". Well, exactly. It won't work.
Reading all your answers here you come across as either very inexperienced or drunk with the prospect of striking it rich or both.