We have a few -200PCFs and a single -200PF with Roller (535-E4). Doing a quick calc for an 1800m rwy (no obstructions) in std conditions gave me a max towt of 105,7 tons. For a 2500nm flight (6 hrs) figure a take-off fuel of around 27 tons. The typical OEW is around 51 tons so you might theoretically get a payload of around 27 tons. Figuring for variations, 25 tons might be a realistic regular load. You might be close to maxing out on volume at that weight, depending on the payload. Not many things will let you get max payload (38 tons on the -200PF) but I have done it a few times.
The converted aircraft e.g. -200PCF (Precision Freighters conversion) have varying weights depending on their original certification. Look closely at these, you want a high gross weight, high landing weight airplane.