polarity reversing & speed control.
For dc motor reversing,you will need to use relays,simple reversing switches can't cope with the ampage,two 5 pin CO relays (16 amp)& a 3 position switch,is all you need.a good source for them is the local car breakers'
For speed control,a PWM is far the best choice,but don't buy one from China (ebay) they will fail because the heat sinks are too small.Build your own.A simple PWM can be populated on a small piece of strip board quite cheaply.
But you must never run it without a heat-sink.Use a LM324N for the driver,& a high powered mosfet,it can be controlled by remote with a pot.
I'VE BEEN TRYING TO UPLOAD PDF DATA SHEETS TO HELP,BUT THEY HAVE FAILED,I WILL KEEP TRYING!