My wife's iPhone 6 works okay with iOS 11, but she is a very light iPhone user. She uses her iPad a lot more. I think her only usage is phone call, calendar, iMessage, takes a few photos/videos every now and then, and some Web browsing and maps/directions. I think her biggest frustration is when the camera doesn't respond right away and she misses an opportunity to capture a photo of our kids. We are going to replace her battery to see if it helps, but it's not nearly as slow as my iPhone 6 was.
I also had an iPhone 6 but upgraded to the X in December. I use my iPhone a lot more than she uses hers. My battery had degraded to the point where it was definitely being throttled as her iPhone 6 is faster and more responsive than mine was before I replaced it. A battery replacement would have resolved that issue, but I wanted a new iPhone anyway.
I would try to get the 6s if you possibly can. Yes, the iPhone 6 will suffice for light use if one does not expect it to be super fast, but the 6s has a much better chance of providing a decent experience with iOS 12. She could avoid iOS 12 of course, but app updates are also going to mean slower apps regardless of the version of iOS used.