Apparently the transmission gears have a max speed of 4, 7, and 18mph, with the same in reverse (don't use 3 reverse)
That's not a tractor transmission but it is just fine for a loader tractor. I'm assuming this is a torque converter with a standard gear shift transmission behind it, not a dry clutch or powershift. I think it would work OK for towing a trailer that's not too much heavier than the tractor, but that doesn't look like it even has a drawbar. You mention a pintle hitch, that might be a good idea, mount that a little closer to the tractor than the drawbar would stick out so you have less sway from the trailer. With trailer brakes, I wouldn't hesitate at all to drive that at 12mph on FLAT or SLIGHTLY downhill, any slope at all and I'd be going 2mph up or down.
That's not a tractor transmission but it is just fine for a loader tractor. I'm assuming this is a torque converter with a standard gear shift transmission behind it, not a dry clutch or powershift. I think it would work OK for towing a trailer that's not too much heavier than the tractor, but that doesn't look like it even has a drawbar. You mention a pintle hitch, that might be a good idea, mount that a little closer to the tractor than the drawbar would stick out so you have less sway from the trailer. With trailer brakes, I wouldn't hesitate at all to drive that at 12mph on FLAT or SLIGHTLY downhill, any slope at all and I'd be going 2mph up or down.