SDK packs allow for the modification of games, the SDK contains the source code of the game. Correct me or add alittle more info if you wish, hehe I dont know much about this sorta thing. And I think seeing theres these pages on the Official Site :
Mods
Levels
Game Editor
You shouldnt have to spend extra for a level editor, Worldcraft was free and whichever program 2015 is using I'm hoping it will be free too.