I'm not sure about the older B7100's but the newer kubota's have a "kit" for the filters which include all the hydraulic, air, oil and fuel needed for your machine. They run about 100-200 depending on model. For the B7100 you'll need another 3-4 quarts of good diesel oil (approx $20). UDT will be the most expensive stuff. Tractor supply sells some stuff that is RATED to be the same as Kubota UDT, and it smells the same, so I think on a machine that old you'd be fine to run it. I believe that stuff is about $60 for 5 gal so you'll have it a while and several changes.
All in, you'll be at about $200-300 for fluids and filters. And your machine will be good for another 100hrs.