Oldest refurbished iPad I should prefer buying?

If longevity is important to you, consider purchase of a current model iPad - as this would be likely to receive ongoing software updates for the longest.

While Apple does not pre-announce end of support, history demonstrates that you might reasonably anticipate support continuing for five to seven years from release of an iPad model to market. Clearly, models released earlier will already be part way through their anticipated support lifetime.

Here is a list of the highest version of iOS supported by the older models of iPad (for simplicity - “generations” are shortened to a number) that are restricted in their ability to be updated:

32-bit devices (obsolete)

iPad1 - iOS 5.1.1

iPad2, iPad3 and iPad mini1 - iOS 9.3.5 (WiFi Only models), iOS 9.3.6 (WiFi & Cellular)

iPad4 - iOS 10.3.3 (WiFi Only), iOS 10.3.4 (WiFi & Cellular)

64-bit devices

iPad Air1*, iPad mini2 and iPad mini3* - iOS 12.5.7

iPad Air2, iPad mini4 - iPadOS 15.8.2

iPad5, iPad Pro (9.7"/12.9" first generation) - iPadOS 16.7.7

(*) obsolete

Newer models are all able to be updated to the current version - iPadOS 17.4.1

• iPad Pro 12.9" (2nd generation and later)
• iPad Pro 11" (1st generation and later)
• iPad Pro 10.5"
• iPad Air (3rd generation and later)
• iPad (6th generation and later)
• iPad mini (5th generation and later)

You would perhaps be well advised to avoid iPad models that are not capable of running at least the current version of iPadOS.

You're very welcome.