Most trailers are rated on weight by x amount per axle. Average for double axle is 3500 per but single axles can vary. Make sure it is attached when you load/unload tractor. On tires, the more ply, the more weight tolerance. 10ply eat cactus for lunch.(what I use). Unless you are limited by room...
When you test drive them, check the shifting. Clutch's can be an issue. When you push in the clutch, it should stop spinning quickly. I would also recommend diesel instead of gas to avoid carb issues. I picked up a Case 685 for 10k with loader and it does all I need. My first tractor was a 70's...