My first seller financing proposal! Feedback appreciated...

I have a seller financing deal I would like to propose to one of my clients to purchase his condo for my own personal property. This condo is more like a townhome, and is actually completely detached with a front patio and large backyard. He is 72 years old and this is a property he has owned as an office since 1999 he purchased for $191,000. He actually lives in a SFH down the street from me and I am going to work with him to buy a larger house later this year. I think he is semi-retired and does not earn much of any taxable income which is why I thought this deal would make sense.

Purchase price: $450,000

Down payment: $90,000 (paid over 3 years to stay below capital gains threshold)

After three years, approximately $1400 a month for a 17 year term to pay the complete balance. 

He will save approximately $80,000 in capital gains taxes, I made the term shorter as he gets older, and he can use the income to justify a small mortgage if needed on his next home purchase. 

Are there any other factors I should be considering for this deal? Thank you for your feedback!