You are allowed 25 there and you will want to go there once you get in to it. But don't discount being able to go up to 30 because it is useful at times to deal with the flow of traffic.
I want to be clear about the drive I described because I am a bit confused by your post. The motor would drive the left side of your hub with a set of sprockets off a gear reduction on the motor that would either favor low or high speed. This is the main reason that people are going the gearing route because they are not interested in pedaling. Anyway I would suggest low in your instance. Your drivetrain would not have to change at all from your current setup as pictured as it would be used to help the motor get off the line and to assist it to the top speed and on hills.
As an aside, the biggest reason that people run stop signs on bikes is because starting from a dead stop is hard. This is causing lots of problems in urban areas. With your motor setup you will be happy to obey the law, even fully loaded, because getting going is easy and actually fun because you know how much effort it has been all these years. 150 mpg+/ gal is also nothing to sneeze at and imagine if even 1% of the population of Portland alone used this type of vehicle for their single person/purpose needs. Less overall energy use, less congestion, ah, one can only dream......or be part of the reality.
