If you are looking for efficient and reliable you will be much further ahead to just buy a common factory made PM motor that has your speed, voltage, and current requirements and use that as a generator.
Through out my years of playing with AE I have came to the simple conclusion that a factory built and re purposed motor beats home brew hands down in cost efficiency and reliability. Its why I use mostly Getty's PM DC servo motors and Fanuc three phase AC servos for my generators now. They take far more abuse than anything I could hand craft while maintaining excellent efficiency and overload capacity.
Being that second hand or surplus commercial PM servo motors come in every shape, size, speed, duty cycle, voltage, and in either 3 phase AC or basic DC its just a matter of looking in the right places to find your match.