If you are thinking about getting a device, I would highly recommend to get a device with the lastest OS updates.
We try to keep these low, but we will raise these, when there are incompatibilities. We have seen issues with iOS 14&15 recently - IOS App im Vordergrund halten, Bildschirmsperre deaktivieren - #13 by boldtrn
In the Android ecosystem it seems to be normal that devices use outdated versions of the OS in the Apple world that is less common. So for Apple definitely the lastest OS, for a list of compatiblie devices see Apples documentation.
For Android, I would at least get an Android 12 devices (I recommend 15).
Regarding the devices specs, our app does not require a lot CPU or RAM, if you don’t mind that the whole phones feels slow, you can get low end specs for Kurviger.
My personal recommendation for Android is a Pixel. Google will provide OS update for 5 years for Pixel 6 and above and 7 years for Pixel 8 and above: Learn when you'll get software updates on Google Pixel phones - Pixel Phone Help
So a used Pixel 6a or 7a would be my goto. Used iPhones are usually more expensive.