DC motor field voltage
I assume that rectified current will be almost constant due to field winding inductance and it is like feeding the winding with average rectified voltage, that is about 200V. Is this assumption correct?

Yes, that's right. When the input voltage drops, the motor field inductance will continue to drag current out of whichever rectifier diode is connected to the highest voltage. The net effect will be as if the field winding is connected to a source with a voltage of the mean DC output of the rectifier.