Sarah, I just have to say how well organized and helpful you are. What a gem!
Also for everybody else to compare notes, here are a couple of other details of our case:
I'm the American sponsor who is a housewife-mom w/ no income. I have to be the sole sponsor on the Affidavit of Support so we are using our assets, which consists mainly of the capital of our house here in the UK (house is currently under offer but we've not yet exchanged contracts).
As I have no income and therefore am not required to pay taxes, I called the IRS helpline. They were extremely helpful and sent me a form letter which states that you are not required to file if your income is below the required limit. I thought this would be helpful to have on IRS letterhead, in addition to my statement, for the I-864.
Tony, my DH, has been to the doctor to get the innoculations required. Amazingly, our GP was able to extract his health records all the way back since the day he was born. The nurse is sending us a letter verifying that the innoculations were administered.
Additional things that I will bring to the interview to help our case are: statements that show we have joint, active credit in the US, US birth certificates and passports of our two children, my old 1040's from before we moved here that show I have the ability to work and earn, Tony's P60's from here that show he's employable, Tony's social security card, contract on house sale (assuming we have one by the interview date), my son's application for school in US, the order we place for the international transfer of funds (again, assuming we have contract on house sale.)
This is probably more than is needed, but I want to be safe rather than sorry.
Good luck!