Buyer looking to do Owner Finance with seller

[Victor Carreon]

I am looking to acquire my first investment property. I currently rent a store front of a mixed use building from a relative. The building has two apartments (1 with tenant who is said to leave this month and the other occupied, both are renting for about half the current rates) and the storefront will be empty as I will be leaving the space for another opportunity. The building is paid off by the owner but wants nothing to do with the property or RE as soon as I leave. I offered to purchase the building from him and he is willing to finance me the deal. How and what do we need to do to put everything on contract and come up with the payment terms, amounts? I would appreciate any recommendations on great attorneys with experience in this around Chicago/NWI. What would be the best way to go about to minimize fees, payments, taxes on both ends? He has owned the property for about 20 years now. He is retired and is also worried about paying capital gains. The building is worth about $150k. He hasn't given me a final sale price but I think it will be reasonable.