Thanks for responses that are pretty clear, even to a beginner.
For the 1N4148 vs. 1N4004 discussion, I'm again poring over the datasheets and wondering what's the critical characteristic for "underrated". Is it Io, Average Rectified Forward Current, which is 150mA for the 1N4148 and 1A for the 1N4004? If so, would any diode do, whose Io was greater than 500 mA?
About the MOSFET. The pump motor says it will switch on at 8V (and run weakly), but it runs best at 12V. Did I hear that the MOSFET will allow all 12 volts will make it from the power supply through the pump?
Please could someone show me how the output from the LM741 is limited to 20 mA?
I was surprised that this circuit has no capacitors. But as I know so little, I didn't want to mess. Are they unnecessary here? If they would be useful, do I go with the rule of thumb "capacitor voltage should be twice the supply voltage" or, in this case, 25V?