Advice needed on potential first fix & flip with seller financing

My neighbor is ready to sell his home. He is older and does not have a mortgage and there is a lot of deferred maintenance on the property. ARV $450k. Thinking of offering seller financing deal to him. Repair costs would be $150k. He is hoping to get $200k but current value of the home is more like $160-$170. We would likely take 2 years to rehab since this is our first one. But I'm not sure everything I need to consider when underwriting a seller financing fix and flip to see if this deal would even make sense. I tried the BP Fix & Flip calculator but it's not really geared toward seller financing. Would love any advice or insights. This could be a great opportunity but I'm not so eager that I would do it if the numbers don't make sense.