Landing weights on 75Fs in our fleet are almost uniform 95,254 kgs, a single one is 89,811. The landing weight can be increased but it involves paying Boeing an arm and a leg and through the nose as well.
With a take-off fuel of 16,7 tons you can assume a 3-3,5 hr sector depending on payload, landing with 4-4,5 tons min.
The RR RB211-535E4 powers later model 752s. The -535E4B powers solely (to my knowledge) the B757-300 (of which there are no freighters yet, but will probably make a good 18 pallet freighter in due course).
I have never flown a 757 with P&W but they apparently burn a little less. These should however not be discounted if available.