I am on it again. After having an idea for the spawning script, I thought about what must be given, to get a good level. I came up to this:
As you can see, there are different values which are important to get an possible level. Also, I will be able to adjust them to increase the difficulty while the player is getting further in the game.
I hope, that I come back with a (nearly) complete spawn script in an hour. See you then. =)