I have one and believe me when I say you don't need one.
There is no magic number or frequency that will break apart the water molecule. Anybody that tells you they have it figured out, is lying or doesn't understand what they are doing. The problem is this: the HHO generator starts at room temperature, and as it runs it will climb. This change in temperature will cause the resistance of the electrolyte to change. In order to build an RC resonant circuit you need to have a stable resistance.
If you wish to use a PWM you are doing so only to limit current to the HHO generator. For this reason you just need a good amp meter to set the duty cycle of the PWM.
However, if you really do wish to buy an oscilloscope, you can get a good 100Mhz model fairly cheaply on ebay. This will handle most of the PWM's out there. The O-scope I bought is one that goes up to 1Ghz and they cost a bit more.
You did say you were interested in motors and such, so you wont need one that goes that high. Most stepper motors have something like 200 steps per revolution, so if you want a motor that spins at say 3000 rpm you would want your o-scope to operate at about 600Khz
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Some days I get the sinking feeling that Orwell was an optimist!