Unity Enemy AI

Hi, I need a script for 3D enemies, I don't need combat, animations or other things, all I need is a script that can be attached to different characters, I need this script to detect when the player will be near it and make the character move towards him and I need a variable that determines when he should stop, based on his distance to the player, this distance will be defined by his attack type so I would like a variable that I can modify , something like an "attack distance".