Be careful with purchasing older Apple products. Many times of the apps will not run on them. Have an old iPad and am experiencing this now.
I don’t use apps, so not an issue for me.
If you want run the latest IOS (Apple operating system) and apps, make sure you get an iPhone 6s or higher. I use Swappa. Phones must be in complete working order and the site will guarantee your $$ if the seller does not abide by the rules. The cheapest iPhone iPhone are from $100 to... 6s, 6s+, 7, 7+, 8, 8+...