How are you structuring the deal..... are you doing a contract for deed or are you buying it out right with a note and deed or trust?
This matters for example of you have the possibility to sell early before the 15 years are up. If the deed is in fact transferred to you then you own the property but have him hold a mortgage on it. Now assuming you sell the property in the 5th year with this scenario where you are holding title and not the owner, then you pay less for the property on the 30K with 7.5 than you do with the 50K at no interest.
It's in the best interest of the owner to structure it with no interest at 50K so if you pay off early he still gets the remaining balance of the 50K. It's a better deal for you if you structure it at 30K with 7.5% if you sell early. All of this is a moot point if you hold the property for the full 15 years.
Let your conscience guide you for what you think is a win win situation for all involved based on your exit strategy too not just the buy strategy.