For a speed controller try www.electronize.co.uk . Look at the FR30HX model. Has reverse, operates at 6 - 24v, 30 amps continuous, 90 amps short term stall. I picked a new one up from ebay for $77 shipped from the UK. Very nice unit. You may want to check out Novak's line of esc. I think the GTX can handle dual motors, but I'm not sure of amperage.
The mechanical 'step' style speed controller that your describing has been around for awhile. They do work, but they are not very smooth in operation, can get very hot even at low amperages, and are not very efficent.