the following pictures show you everything you need.
the first picture shows everything you need to do in the door bp. create the variable, click the eye icon, set its type, and then use the variable as needed in a script. in this case i included on overlap check if the overlapping actor is the player, then open the door, and when the door is open then open the next level which is based on the variable.
the second picture shows how to set the variable on a per instance basis via the details panel of the level editor. you can set teh variables value for each door to something different for each one. so one door could go to level 2 while another door next to it could go to level 6.
