Have you tried calling Cannondale? They might be able to steer you to another shop which is more helpful.

You have to look at things such as road or mtbike. Size of the wheel (700, 650, 26", etc.). Is the cassette OK? Are the rear drop outs and derailleur hanger OK? If your rear derailleur is damaged, I'd want the frame checked also. You really need someone to lay eyes and hands on the bike to give you an expert opinion on the damage.