Two theoretical question about my PWM motor driving circuit

I have two theoretical question about my circuit (that I have tested with a breadboard and it works, but I want to understand better some design choices

1) As far the flyback diode is concerned, I have at my disposal 1N4007 high voltage diodes, that can be good because a 10x reverse voltage is suggested in these applications, or 1N5819 shottky diodes(datasheet: http://www.kynix.com/uploadfiles/pdf86758/1N5819.pdf ), that can be also good for the short reverse recovery time. The motor work at 12V, absorbing 5A. Between these two, what should be the best choice?

2) I have to determine the maximum frequency that I can use to drive my circuit, allowing the MOSFET to turn on and off completely without producing additional heat. The PWM circuit first goes into an ULN2803 darlington array, and then into an IRF9540 power mosfet. Considering the delay related to the two components, how to determine such a frequency? (I'm interested in how to compute the theoretical value, but working with a 25Khz PWM I shouldn't have problems, right?)