MuttDriver:
I’m now thinking of using these for flyback diodes: http://www.sanrex.com/images/PDFs/BKR400AB10.pdf , but I can’t make out if they are fast enough. Would it be a good choice? Or can anyone suggest an appropriate diode?
I would go for something a little smaller, perhaps http://uk.farnell.com/stmicroelectronics/stps200170tv1/diode-schottky-2x100a-170v/dp/1295321 or http://uk.farnell.com/genesic-semiconductor/fst16035/diode-rectifier-35v-160ato-249ab/dp/1862671. Although it needs to handle a peak current of 200A (or whatever the motor stall current is), the average current will be very much less. Unfortunately, reverse recovery time isn't quoted for any of the high current Schottky diodes I found.
MuttDriver:
I’m planning of using the mower chassis for common ground (like most cars). Is that a bad idea?
Using the chassis as ground is OK for accessories like lights, but a bad idea for signal connections and power connections.