No ESC's do not need any MHZ. Only Radios do. For an ESC, you would need to get one suited for the type of motors your running. There are two main types of ESCs, Brushed, and Brushless. These ESC's vary in Amps. your motor will draw a ceartain amount of amps, and your esc will need to handle that much. Say you are running a 550 motor that pulls 25a. You would want at least a 30, or 40 a esc, so when you get seaweed stuck in your prop (harder load on motor) and it shoots the amps way up, you will be ok.
Dan