How far back can I go if I want to save going with a refurb?
I dunno about answering your question as such, but general advice here... If you're able to buy a refurbished as-new unit it'll probably be OK, but I'd caution against buying a device which is a number of years old already because the battery ages and I believe Apple has implemented its clockspeed cap thing in ipads as well because my Air 2 (which I bought new around when the device first launched) is now seriously slow.
Apps which launched almost instantaneously now take several seconds to load up. Even unlocking it is much slower than it used to be. And of course, the battery only holds charge for a few days now even if it sits in sleep/standby mode most of the time.
My dad's ipad air 2 - which was newer than mine but which he also used a lot more than I've used mine - died a few days ago after also having suffered severely shortened battery life. It's now stone cold black screen, and won't turn on even if connected to a power source. *shrug*
So don't buy something that is
actually old.
Could be you won't get as much use out of it as you might have hoped...