Aaglo - the Doom Bell could work as Celeborn suggested above, but either way, it isn't really one of the main war machines of the Skaven. More typically they use the a gatling gun called the Ratling Gun, the Warpfire Thrower which spews green flames at foes, and the Warp-Lightning Cannon, which I think shoots something akin to a laser beam. You're definately right on their war machines being unique, but isn't unique good??
As for the race's necessity, they are no less necessary than a number of the civs in Warhammer. Norsca, the Amazons, the Ungols, the Dolgans, Lahmia, etc... these civs don't even merit actual armies in the Warhammer game. The Skaven, on the other hand, have a wonderfully diverse army that includes everything from technological war machines to ninja-like assassins to mutated monstrosities to fanatical plague rats.
The Skaven are constantly on the fringe of things in the Warhammer world, plotting in the shadows and seeking to take advantage of the weakness of others. And the location of their capital, in the heart of the Old World, puts them in a position to really take advantage of any weakness Bretonnia or the Empire might show if faced with wars that draw their forces away. I think they could really add a nice aspect, especially to a game on the WH map, in which the great Empire and other human nations always have to hold back troops in order to protect against the potential of a sudden Skaven invasion.
Celeborn - Great job with the model! I'm looking forward to the addition of the Skaven hordes to my game.