The fatter the unit is, the quieter it's likely to be. A good garbage disposal will run anywhere from $100 and up to $300 - anything cheaper is not worth buying.
There are two types of disposals: continuous and batch-feed. Continuous-feed models are activated by a wall switch. Batch-feed models start up when you turn the lid. Batch-feed types are quieter and safer, because they can't be run unless the lid is in place. The mechanism also prevents silverware from falling into the running disposal. However, they're less convenient and come with fewer options.
A disposal should have its own 120-volt electrical circuit. The connection can either plug-in or hard-wired, though some communities require hardwired. Check your local code before doing anything - some building codes prohibit the use of disposals, while others require them.