Assuming that you want to play Gwent on it because you have asked the question on the Gwent forums, let me list the supported devices:
Please make sure that your device runs iOS 12.0 or newer. The supported devices are iPhone 6s, iPad Mini 4, iPad 5, iPad Air 2, iPad Pro, iPod Touch 7 and newer ones.
So, any iPhone 6s or higher with iOS 12 should do the trick. If you can get a better one cheap, go for it. However, my personal advice is to wait for the Android release and get a (cheap) Android phone, instead.