If you are talking a Series 2, then you are looking at the Used Apple Watch market, unless you already have a Series 2 laying around the house.
The iPhone 6 can pair with an Apple Watch running the older watchOS 5 software, however, the Series 1, 2, 3, and 4 used Apple Watches can be upgraded to watchOS 6 which cannot be used with the iPhone 6.
So in theory you can pair a used Series 1,2,3 or 4 model still running watchOS 5.
But if the Apple Watch is still registered with the original owner's Apple ID, it cannot be unlocked by anyone, so if you are buying a used Apple Watch make absolutely sure it is unlocked And you can contact the original owner to fix up any issues, as there are many reports of people sending the money and the seller ghosting away so you are left with a locked and useless Apple Watch.
If you get a used Apple Watch make sure it is running watchOS 5 and the original owner has followed these instructions:
What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else
https://support.apple.com/en-us/HT208855