iPhone 6/6S is only 2 to 2-1/2 years old.
Apple supports their iOS device for 4-5years.
So, an iPhone 6, right now, is approx. halfway through its cycle.
Purchase the phone new or refurbished through Apple.
https://www.apple.com/shop/browse/home/specialdeals/iphone
Apple treats ALL their certified refurbished products like new, with all the same warranties applied.
Currently, Apple ONLY offers the iPhone 6S and 6S Plus as new.
Apple, also, still sells the, iPhone SE.
DO Not purchase an iPhone 6 used unless you buy it in person and is NOT Activation Locked by some unknown user where you will never, EVER get the iPhone to work!
If you want to purchase one, this is totally up to you.
Just realise that you may only get one or two more iOS software upgrade cycles left for the device.
Get one with the largest storage amount you can afford.
Budget for 2-year extended AppleCare which you can purchase at the time of purchase or within 60 days of initial purchase.
Good Luck to You!