Level select design approaches.

I need some advice regarding level select design. I have five separate 3D levels in my game - I will have several objectives the player can complete in each level (different objects and characters appear depending on which objective the player is attempting). I plan to have a GUI screen full of buttons (arranged by level of difficulty) - each button corresponds to a specific objective in a specific level (e.g., 15 EASY level buttons on Level Select Screen 1). How should I go about managing each objective for each button? Should I have a separate script to handle each objective (i.e. 15 separate scripts for each button in Level Select Screen 1), or should I have 1 script for each level and handle each objective within that level's script? Also, since different things appear depending on which objective the player is attempting, should I just instantiate each objective-specific object into the scene once I load the level (the only way I know how to do this is to have an empty game object already in the level, and instantiate the object in the empty game object's position)?