I know plenty of people who tow 10K with 3/4ton trucks and have for years without ruining the truck myself included..
I notice there are only two stake pockets for 10 feet. I would recommend getting a steel U bracket bolted on/welded half way down to keep the wooden sides from flexing or breaking. Also one thing to think about with sides is if you load the trailer with a tractor/skid steer at all. I only put 20" sides on mine so I can still load it with a small tractor/skid/miniskid if I want, and then I have the 4 foot sides for using it with the leaf vac.
Peter when you say rust do you mean the whole thing is rust like an old dump truck, or the areas where something like stone put a good scratch in is rusting? I'm pretty sure its normal to have some rust in the bed of a non aluminum/stainless dump after a decent amount of use.
"Democracy is not a choice between good and bad, but between bad and worse."