(Please click through to Think Geek. They might be having a sale!)
The basic "Red Box" set is where your characters got their start, but if you want to bring them up to levels 4 through 14, you needed the Expert Set expansion. This is where the game really started to get good, with flashier spells, more monsters, more detailed equipment, the ability to attract NPC followers, wilderness adventures, and long-term campaigns. Oh, and The Isle of Dread module that took us on a journey to recover lost treasure from a prehistoric island...