It’s a difficult quandary honestly...
If you plan to update iOS (or, well, iPadOS), you would want the newest model and battery you can get, and the most RAM you can get. If possible, the 4th-gen (2020) iPad Pro would be a good option, too.
If you play to stay behind, then the new battery model would guarantee maximum longevity.
That said, maybe, like we have stated earlier, the A12X gets to be the first model with great longevity, and if you don’t require the battery life, it should work. iPads’ battery longevity (like iPhones) is massively variable, but a new battery would guarantee more longevity, obviously.
2GB RAM iPads, for instance, were quite criticized here after iPadOS 13. I have mine on iOS 12 so I wouldn’t know, it runs quite flawlessly, but that’s something else to consider, too. RAM has historically been quite important throughout the history of iOS updates.
Assuming battery life on the cheapest iPad you mentioned is fine, I think it’s a pretty good price, and if iPadOS updates aren’t as impactful as they were due to sheer power... it could be a good option. However, like I said, a new battery would guarantee more longevity in that regard, especially considering what
@Digitalguy said about the stellar performance of the A12X.
As you can see, I’m not entirely sure what I’d do myself if I were you. Maybe the rest can weigh in with their opinions?