I'm surprised you can even find an iPhone 6. They were end of sale/discontinued in 2018. And...
As of March 2026, Apple has officially declared all models up to and including the iPhone X obsolete: original iPhone, 3G, 3GS, 4, 4S, 5 (added March 16, 2026), 5C, 5S, 6, 6 Plus, 6s, 6s Plus, SE (1st generation), 7, 7 Plus, 8, 8 Plus, and iPhone X. These models can no longer be repaired or updated by Apple
So sounds like you'd be buying a brick at this point.
If you are looking for an inexpensive option that will not have a cellular plan tied to it, try Ebay for an iPhone 10 or newer and just use it with WIFI. There are tons listed for less than $100.