We do not have app-specific requirements. We simply require iOS 13. This page shows devices compatible with iOS 13:
https://osxdaily.com/2019/06/04/ios-13-compatible-iphone-ipad/
Every Fall, we increase the requirement by 1. For example, by the end of this year, we will increase to iOS 14 minimum and add iOS 15 support. For older iPads, the last compatible software version remains available to you after such a requirement increase.
When setting up a new system, my advice is always to use the newest/best iPad available even if it seems like overkill. Given that the typical use case is dedicated devices, the typical usage pattern is to purchase a new device, place it into service as a dedicated Roomie device, and give it 4 years until trade-in and upgrade. If you start at the end of that sequence with a 5 year old device, you can probably keep things working for a while, but it’s going to be difficult and certainly you’ll need to prevent iOS upgrades – assuming there are any for such an old device.
For new devices today, the iPhone SE 2 is currently the best price/performance for a dedicated device.