The 200 watt rating near the power cord is the rated power consumption of the amp, but actual power consumption can be higher if the amp is run past its rated limits, as when pushed to distort. I think a 500VA transformer is a good choice. I agree that 50Hz/60Hz is not an issue.
The polarity switch reverses the polarity of the chassis ground. This was a useful feature back before electrical polarity was standardized, but serves no useful purpose today. In fact, it connects a capacitor from ground to power. If that cap shorts, it can be dangerous, causing electrical shock and even killing the user. That's why it is often called the death cap.
The electrical outlet under the circuit breaker is an accessory outlet, used to plug in another piece of gear.