Do you think I'll need to limit the voltage across the buzzer (2 (/3)
diodes in series in parallel with the buzzer?) and that the existing
bell draws at least 25mA?

That would depend very much on how long your delivery individual holds down the button. But a couple-of-ohms, 1-watt resistor, or so, in parallel to the buzzer should give all the protection you need. One watt is crazy-overkill, but cheap enough.

Peter Wieck
Melrose Park, PA