For the ultimate car-replacement vehicle, a STEEL cargo bike is the way to go. I have an aluminium bike (Kona Ute) because I need to be able to pick it up and carry up stairs daily, but if weight is no concern, go with the heaviest most rugged longtail frame you can get. Many have used Yuba Mundo with best of luck, and you can find a very detailed and overall excellent build thread here on ES.
You will also need custom wheel builds done with 10 to 12G spokes and bored rim eyelets using the strongest DH rims available. From my experience, I strongly recommend using Stan's rim strips and sealant. No matter what I did I never got a flat with them. Even cracked and bent a rim, sprayed the goo all around on my DH bike, but the strip still kept the air in my tyre.
Don't worry about front traction on a cargo bike. I've had zero problems on the Ute. Use the motor you already have up front, both DD and geared will be fine if you use enough torque arms and a tough fork. Give the rear DD 50% more amps than upfront. As the rig becomes heavy, you realize the extra 2kg don't make much difference, so you can also make a dual DD rig like I did a year ago. I also used it to carry my 100kg brother on the odd alcoholic occasion, and it did fine.
The strategy should be both motors on anytime you need serious power. You want to share the load between motors so you rarely slip out of the motors' efficiency zone. About the only time you turn the front motor off is for cruising on flats, or when you need to be stealth. As stealth as one can be riding such a bike.