Seller financing from relative in context of CF/inheritance

[Lee Carter]

My father owns a duplex outright in Washington State that cash flows him $1200.  He wants to maintain that throughout retirement, which begins next year at age 65.  I am 43 year old and life expectancy in our family is 85 years old. As he ages, he won't want to self-manage or even deal with the place.  Upon his passing, I will be inheriting it.  However, he recognizes that selling it may bring in $120k now post-capital gains taxes, which would be nice to start retirement out with but has a limited shelf life if he lives on the principal ($120k/$1200 = 8.3 years of cash flow if he still owned it).  He doesn't want to 1031 exchange it for anything else.  I'm considering buying it from him now.  If we do 150k @ 5% interest (selling financing), the payment would be around $1200 to him for 15 years and no hassle of managing.  If he wanted the full balance for whatever reason along the way, we could structure it accordingly.  This would help to avoid taxes on his end.  

Any thoughts on the potential upsides or downsides that I'm missing?  What other considerations would be beneficial?  Is there any benefit to buying it now versus inheriting it for free later?  Tax issues?