I've pulled 10.5T on 3.5T trailer. Handling is good. Stops are good, even panic stops, even with e-brakes. Air brakes are a better way to go. Power is the problem. 135 hp doesn't cut it on grades. Going faster that the max load rating (45mph) is not an option. In my opinion the chassis,brakes,pintle hitch,tire load cap,cooling sys. all seem fit for the task. That low towed rating -may- be for the small engine. The trans. seems small (physically). I always shift with great care. NDCC's may be a concern in the rain. Extreme duty - so maintain well.
Tail wagging is a sign of too little tongue weight.
Ace is right, pulling 10T best left to the big boys.
JimK
p.s. Wanted to buy- M923