Will an Apple Watch Series 2 work with my iPhone 5s?

You should, as long as the device has not been updated to the latest watchOS. Since Apple is no longer selling the Series 2, I'm guessing this is a used device. I would check with the seller to see what version of watchOS is on the device.

It depends on what firmware version is on the watch,if it is watchos6 then NO it won't work,if it has watchos5 or lower then yes it will work.