You need to go through a closing attorney before you get into trouble. My seller finance deals went through the normal closing process. Since you are helping him out then I would only put down 20k
I agree with Sedgrid, close with a real estate attorney and/or a title company to make sure all the paperwork is done correctly, and filed, to protect you and the seller.
For the terms, there is always some give and take between downpayment, sales price and interest rate so it will depend on your degrees of flexibility as well as the sellers.
Commercial financing is sometimes assumable but if the loan(s) are 30yr fixed, residential type mortgage they would be subject to due-on-sale.
You could execute this as a wrap mortgage where the properties are sold to you, the seller continues to pay his underlying mortgage and you create a promissory note between you and the seller to document the financing between the Seller and You. There is risk to triggering the due on sale clause but typically, as long as the bank continues to get paid, a bank will not go out of their way to disrupt a performing note.
If you have never done a wrap seller finance transaction before, you will absolutely need a good real estate lawyer or title company who is familiar with these transactions. You will also need a good insurance company to help structure the insurance correctly. A good source for these types of contacts can be the local real estate networking groups in your city.