Personally I wouldn't go earlier than AP 2.5 (August 2017) unless you don't care about EAP features at all. Both AP1 and AP2.0 have already been made obsolete with the v9 update and will continue to miss out on future updates as the tech progresses.
MCU2 vs. MCU1 is another big change which happened in March 2018. If you have the older MCU you aren't missing out on any features (yet) but you will definitely notice a slower response time on the touchscreen.
In terms of build quality, it seems like most of the major issues were fixed by mid-2017. Of course there are still problems, but nothing like the earlier models. I would definitely avoid anything with a build date of 2016.