If your 760 w/ 250k miles is running fine it probably would be alot cheaper to maintain than a 850 w/ 200k miles. You also need to determin the "certain interval" needed to add oil. Is it every 2000 miles or every 200 miles? And keep in mind that the present owner will probably estimate this on the high side. Is it burning or leaking or both? Either way could be very expensive to fix. If it's burning oil alot could be sign of a marginally maintained engine although some extra oil usage is to be expected w/ high mileage. 850'S in the 100-125k mile range can be found.