First seller financing deal

[Slaughter Paul]

I came across a three bedrooms, two bath single family property that I believe needs about 30k in rehab. The seller bought the house at auction but do not want to deal with it right now because he got other properties in front. He is asking for 65k with a seller financing option. My next step is to visit the property and take as much pictures as possible to have a contractor estimate the rehab cost and timeline before making an offer. After doing my due diligence, I found 10 similar properties on Zillow with an average sale price of 114k. My strategy is to use the BRRRR method for this property but I don't know the ARV of the property. However, Zillow estimate the house at around 105k.

How do I negotiate a favorable deal? What to look for in a seller finance deal for someone who has no experience?