There are many options for driving DC motors with variable speed control, more than you'll likley every want to know about. I like the autotransformer and bridge rectifier sollution though, it's very simple and would be very reliable if you properly over rate the diodes. The inductance/intertia of the motor will do some smoothing itself but you still may get 120hz pulsations without capacitors to filter the output. A dimmer switch will work if you can find one rated for the current, but it's output is pretty rough so the filtering requirements are going to be higher.