Getting an iPad that runs the current iOS is always a good idea. If money is tight you can get an iPad Air 2. That's the device I currently use for backwards compatibility tests and it is capable of running iOS15. I will try to support iOS15 for at least 2-3 more years. Currently you need at least iOS13 but any device that can run iOS13 can also run iOS15 so there is no real reason that prevents updating the requirement to iOS15.