The MacBook Air series, since 2010, is actually a decent machine. Personally I think they entered their golden era from 2013 - some would suggest 2012, but I came across some issues with that model year (speaking personally at least), and the 2013 revision added PCIe based SSDs that improved the read and write performance over previous models.
Despite being a thinner machine, the batteries in these are easily replaceable. You remove the bottom cover (10x Pentalobe P5 screws), remove 5 Torx T5 screws from the battery, and disconnect the cable from the Logic Board. No glue, no heating, only simple screwdrivers are needed. It’s the same procedure for all models, from 2010 to 2017, both 11-inch and 13-inch. Adhesives were only introduced in the newest 2018 redesign.
Removing most components is that simple, especially on the 13-inch model. Once the Battery is removed, the Fan, Logic Board and I/O Board are only a few screws extra.
For a second-hand machine, a good condition 2013 to 2017 model is a reasonably safe bet. They’re new enough to still be well supported, and supplies of consumable components are still guaranteed for years to come - at least in the aftermarket, as Apple is narrowing the “Vintage” window a little (5 years from last date of manufacture), and although a 2017 battery is directly compatible with a 2013, due to internal policies they won’t sell you one. So essentially if you want support at AASPs and the Genius Bar, stick to a newer model if possible.
The 2017 model is the same as a 2015, even down to the same part numbers, motherboard identifiers, Intel CPU revision and the boards are directly interchangeable. The only differences are the i5 model received a bump in clock speed from 1.6GHz to 1.8GHz, and 8GB RAM was standardised across the range.
If possible, I’d aim for somewhere around a 2015 or newer.
That said, 2013 and 2014 models are also good machines, and it’s possible you may be able to find some better deals on them, being a somewhat older model. Just be aware that eventually when Apple soon stops servicing them, some DIY maintenance may be required when it comes time to replace batteries and the like.