If that truly is the only purpose, then any good condition 2015 or later model should work just fine. 2015 had the best 24 kwh battery packs and the 2016, 2017 had some hit or miss 30 kwh battery packs, be careful with those. Then move on to 2018 or later and you can't go wrong provided the battery wasn't abused by the previous owner. The same can be said for the earlier model years too. That's why LeafSpy is such an important tool. The capacity bars on the Leaf dash are just "general", no context information, but you want to make sure the battery pack isn't victim to some bad cells that would cause you headaches in the future.