> What are the parameters that decide the selection of a freewheeling 
> diode? If the motor is rated 8.3A @ 12V, should the FW diode be
> rated more than this?

Mohit, from my limited experience with motor drives, the internal diode
is intrinsic to the fabrication of the FET. The MBR3045 is a Schottky
diode and conducts before the FET's internal diode, taking at least the
bulk of the back EMF. So you should be looking at the forward voltage
of the external diode (ie lower than that of the internal diode) and its amp
rating. The IRF4509's diode is said to have a Vf of 1.6V max at 38A

Can't help with the calculation for the diode, though I suspect the back
EMF would be equal to or less than the drive, but can't support that with
V and I figures

I'd bookmarked this page at some time

http://homepages.which.net/~paul.hills/SpeedControl/MosfetBody.html

Try this

http://www.irf.com/technical-info/appnotes/an-967.pdf

I believe one reason to use an external diode is to share the heat load,
ie instead of the FET's diode producing heat, that is dissipated by the
external one

http://www.mcmanis.com/chuck/robotics/projects/esc2/FET-power.html

The other protection you can add is a Zener diode clamp on the gate

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