Any Danger?.
Most of the danger is related with your security practice (downloading cracked APK from unknown website, giving application all permission, etc.). On a side note, many banking application would refuse to run on rooted device.
in addition, rooting will make it impossible for you to automatically update OTA from the company, and security patches, that will make your phone unsafe.
FWIW, many Android smartphone only get update for 2-3 years after it's released. Since OP own used phone, i doubt there's any update for that phone.