I think you might be underestimating your power requirements, at least for the cutter. According to their spec sheets, the motor that spins the blade on an cordless electric lawn mower is 24-60V and uses 600-1000W. I doubt they would use a motor that big if they didn't need to. Higher voltage will obviously keep your amperage and wire size down to something manageable but it will still generate some serious heat. The batteries in the commercial units weigh 30 lbs or so and are usually several 12V batteries in series.
Once you add all that weight, your drive motors may need to be bigger, and the load on your steering joint will be obviously higher.
Wet grass is also pretty slippery so in addition to the torque , you will also need some good traction to make it up a 30 deg slope.