I would suspect failed diodes within the rectifier pack on the alternator. As a 3-phase unit there should be 6 of them. If one diode fails the alternator lives on borrowed time as it overloads the other diodes until another fails, then you lose a phase and its game over.
The other problem could be failed windings (unlikely) or a lack of excitation from the voltage regulator, requiring a new regulator to be fitted.
Many auto electric specialists should be able to fix the diodes or regulator, and there's no way I'd be happy to pay that amount quoted for the new one, its just shocking.
If I had to buy a new alternator, I would go for the later version with the greater output capacity.