Oops, I saw the hall effect verbiage and assumed it to be an inductive pickup. I see that it is actually in series with the circuit, sorry about that. I think I'd look for an inductive pickup sensor if it were me.
Your motor rating is only 3kW (4HP) assuming 100% efficiency. This seems far too small for a lawn tractor.
The motors are replacing a 13hp ice motor (maybe the right term is ride-on lawnmower rather than tractor). So about 4 hp for drive, which I think is more than enough, and about 4 hp for the blades which might be too little, but if it works I'll probably be happy to cut the grass before it gets to long. With this setup, I can also eliminate some pulleys and the clutch, which hopefully will reduce the power requirements a bit. I'm going to use four 85ah marine batteries, and I'm hoping to get a couple of hours juice for just driving, and maybe 40 minutes of grass cutting.
The Arduino will try do reduce power to 130 amps per motor, and will allow 150-200 amps for max 5 seconds, and will shut of power completely if consumption is more than 200 amps for one of the motors.