To give yourself the best chance of getting longevity of support, you are better off buying a pad that has the latest software – currently i(PAD)OS 13.
iOS 14 is just around the corner though, so it’s possible older pads won’t support the next iteration.
The table for those supporting iOS 13:
iPad Pro 12.9in first, second & third generation.
iPad Pro 10.5in.
iPad Pro 9.7in.
iPad Pro 11in.
iPad Air 2.
iPad Air 3.
iPad fifth generation.
iPad sixth generation.
iPad seventh generation.
iPad Mini 4.
The cheapest way in would be a secondhand 5th gen iPad or iPad Mini 4th gen. Either privately or eBay from refurbishers. Loads to choose from.