According to the news report from iPhonesoft.fr, it states Apple iOS 13 may drop support for the iPhone 5s, iPhone SE, iPhone 6, iPhone 6 Plus, iPad mini 2 and iPad Air. Although the refurbished iPhone 6 price is attractive, interested customer should consider the future iOS upgrade and support before move ahead. That makes refurbished iPhone 7 would be the cheapest option to consider.
It is a surprise given that iOS 12 still supports the iPhone 5S, so that means Apple would be killing two generations of iPhone in one release for the first time.