hi
solid state relay's won't work unless they are the Mosfet version... make sure to get one that is specifically deigned to switch DC loads. The ones that use SCR's inside of them will just latch in the "on" state.
Crydom makes a DC output solid-state relay that switches 20A, with on time of 100 Us and off time of 1Ms:
Crydom part D1D20. Digikey has it.
You might want to check out the electronic speed controllers at the hobby stores, the ones used for radio-controlled cars. they take a servo signal input, and the output can control the speed of a high-current motor. About $50.
You could also use a big transistor, but you'd have to have really good noise clamping. Actually, you have to have really good noise clamping for any switching method! Big, fast diodes (Schottky, 10A, 200V), a few bypass capacitors (1000UF, 35V & .1UF, 35V) and maybe even a choke in line with the power supply to the motor.
D