Jorge, good luck! As far as being able to look at and evaluate the business, we would not be able to confidently and competently do that without actually seeing Tax Returns for the past three (3) years. But you really want your own Accountant to do that, anyway.
As for your question about the Down Payment, the Lender will want to see that you have money in the deal; however, the majority of Lenders will allow you to use the Seller's Note for part of that Down Payment, and most will actually prefer it. Usually, for whatever amount they require as a Down Payment, they would want you to put in half of that. That is not always the case, but it is the norm.
There are two (2) cautions. First, make certain that you can afford to make monthly payments on both Notes, out of the projected Cash Flow or Net. Getting too much in hock, without sufficient, remaining Cash Flow to maintain the business, account for periodic down times or need for funding repairs and such, is the most common failure of small businesses.
Second, the real difficulty is that the Lender may want the Seller to hold a note that has an equal term to its own. So, if the SBA is going to make you a Loan on a 10-year term, it may require the Seller to hold a note for the same 10 years. That could be a problem if the Seller is stuck only on a 6-year term, and you will want to make certain of that, before you go too much further. One way to combat that is to ask the Seller to take a loan based on a 10-year term, with a balloon payment at the end of the 6th year. Then, at the end of that 6th year, you would need to completely refinance the debt, which should not be a problem if you have had a good payment record and the business is continuing to be successful.
If you have a Seller that is being that accommodating, there are a lot of ways to go forward.