Chris, my 2 axle trailer for my 8.5mtr weighs 640kgs.
You will need low ratio if the going gets a bit sticky. A torque converter can only cope with a certain speed difference bewteen its input and output. Under normal circumstances this limit is not reached, but it can be if the vehicle is bogged, eg. it has sunk in sand. Much the same way as a clutch can only take a certain amount.
The in/out speed difference, which causes the converter to stall, is caused by the load on the output being too great. This allows the engine to speed up the input without the output turning. The low ratio will lower the load on the output of the converter and it will prevent the problem.
With a clutch it's the other way. The clutch slows the engine until it stalls. Or the clutch fries
It is easy to get stuck in soft, dry sand, the slightest wheel spin and you'll go down. Also, the sand will constantly form a wedge in front of the tyre and you have to climb this. Very much like never getting over the hump when your boat is about to plane. It saps a lot of power.
I hope this clarifies things a bit for you.