If you purchase the oldest iPadOS 14 supported iPad ( like the 2014 iPad Air 2 and 2015 iPad Mini 4), these are at the bottom of Apple's iPadOS 14 supported iPad hardware list and you would risk that such old iPad models may NOT be eligible for the next major iPadOS 15 release in the Fall of 2021.
Apple, typically supoorts their iOS/iPadOS for 5-6 years and both the iPad Air 2 and iPad Mini 4 will be at this support threshold.
If you are going to purchase an older, but iPadOS 14 supported device, you may want to look a something a little newer, like older iPad Pro models, like the 2015/2016 iPad Pro models or even the 2017 iPad Pro models, or even the i2017 Pad 5th or 2018 6th generation or even the 2019 iPad 7th generation, which has a nearly 1/2 inch larger diagonal screen.
The newer the iPad, the longer its support may be from Apple.
If shopping for a used iPad online, you should purchase used Apple iOS/iPadOS devices from better known, trusted Apple product/device retailers/resellers, capable of running the latest iPadOS versions.
If looking at Apple devices on eBay, get visual ( photo or video ) proof the the iPad, in question has been properly reset for sale and does NOT have User Activation Lock.
DO NOT take the eBay seller's word that the iPad ( or iPhone ) is free of user activation lock!
If the eBay seller is honest, ask them for the visual photo/video proof of the Apple device being activation lock free.
A properly reset iPad powers up to a multi-language, animated “Welcome” screen that indicates that the iPad has been fully erased and reset and has NO user activation lock.
If the photos or video shows some other screen other than the welcome screen, pass on the iPad!
Best of Luck to You!