New investor... First property

[Zapata Tate]

Hello BP family. I'm working on my first deal, but I'm stuck. The owner is 78 years of age and is ready to get out (He has several properties). I'd like to try seller financing, but I'm not sure how to approach it. The property in question is occupied and has had the same tenants for years, but it needs some serious repairs. If I paid the seller's price and did nothing to the property, it would cash flow between $300 and $400. That's a minimum 20% ROI. I've heard of people walking away from the negotiation table with cash from seller financing deals... HOW? I've also looked at hard money. Should I have a realtor involved?