Advice to set up Seller Financed deal

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Hey guys,

I have a property lined up that seems to be a pretty good option for seller financing. The owner owns the property outright. It is a 4 unit apartment building, all four 1bd/1ba. Anyways, the reason I got interested in this deal is because it looked neglected and had a for rent sign in the front. So i called the guy and asked if he was invested in selling and he told me anything is for sale for the right price. 

I just don't know where to start with setting up a seller financing deal. I know that first i need to figure out how much he charges for rent but it was tough getting any real numbers out of him. He's running the place like a mom and pop shop. One of the units he is renting out as a studio, as a favor to one of the long term tenants. Then two other units are empty, one needing work to get to rent quality. I know there is opportunity for big value add here seeing as this place is located right across the street from a big university. The owner is old timer kind of guy and I can tell he is starting to get burned out. So i need some advice on how to handle this and create an opportunity for a good seller financing deal.

Any advice would be great!